This patch introduces p2m_set_range() and its core helper p2m_set_entry() for
RISC-V, based loosely on the Arm implementation, with several RISC-V-specific
modifications.
The main changes are:
- Simplification of Break-Before-Make (BBM) approach as according to RISC-V
spec:
It is permitted for multiple address-translation cache entries to co-exist
for the same address. This represents the fact that in a conventional
TLB hierarchy, it is possible for multiple entries to match a single
address if, for example, a page is upgraded to a superpage without first
clearing the original non-leaf PTE’s valid bit and executing an SFENCE.VMA
with rs1=x0, or if multiple TLBs exist in parallel at a given level of the
hierarchy. In this case, just as if an SFENCE.VMA is not executed between
a write to the memory-management tables and subsequent implicit read of the
same address: it is unpredictable whether the old non-leaf PTE or the new
leaf PTE is used, but the behavior is otherwise well defined.
In contrast to the Arm architecture, where BBM is mandatory and failing to
use it in some cases can lead to CPU instability, RISC-V guarantees
stability, and the behavior remains safe — though unpredictable in terms of
which translation will be used.
- Unlike Arm, the valid bit is not repurposed for other uses in this
implementation. Instead, entry validity is determined based solely on P2M
PTE's valid bit.
The main functionality is in p2m_set_entry(), which handles mappings aligned
to page table block entries (e.g., 1GB, 2MB, or 4KB with 4KB granularity).
p2m_set_range() breaks a region down into block-aligned mappings and calls
p2m_set_entry() accordingly.
Stub implementations (to be completed later) include:
- p2m_free_subtree()
- p2m_next_level()
- p2m_pte_from_mfn()
Note: Support for shattering block entries is not implemented in this
patch and will be added separately.
Additionally, some straightforward helper functions are now implemented:
- p2m_write_pte()
- p2m_remove_pte()
- p2m_get_root_pointer()
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes in V4:
- Introduce gstage_root_level and use it for defintion of P2M_ROOT_LEVEL.
- Introduce P2M_LEVEL_ORDER() macros and P2M_PAGETABLE_ENTRIES().
- Add the TODO comment in p2m_write_pte() about possible perfomance
optimization.
- Use compound literal for `pte` variable inside p2m_clean_pte().
- Fix the comment above p2m_next_level().
- Update ASSERT() inside p2m_set_entry() and leave only a check of a
target as p2m_mapping_order() that page_order will be correctly
aligned.
- Update the comment above declaration of `removing_mapping` in
p2m_set_entry().
- Stray blanks.
- Handle possibly overflow of an amount of unmapped GFNs in case of
some failute in p2m_set_range().
- Handle a case when MFN is 0 and removing of such MFN is happening in
p2m_set_entry.
- Fix p2m_get_root_pointer() to return correct pointer to root page table.
---
Changes in V3:
- Drop p2m_access_t connected stuff as it isn't going to be used, at least
now.
- Move defintion of P2M_ROOT_ORDER and P2M_ROOT_PAGES to earlier patches.
- Update the comment above lowest_mapped_gfn declaration.
- Update the comment above p2m_get_root_pointer(): s/"...ofset of the root
table"/"...ofset into root table".
- s/p2m_remove_pte/p2m_clean_pte.
- Use plain 0 instead of 0x00 in p2m_clean_pte().
- s/p2m_entry_from_mfn/p2m_pte_from_mfn.
- s/GUEST_TABLE_*/P2M_TABLE_*.
- Update the comment above p2m_next_level(): "GFN entry" -> "corresponding
the entry corresponding to the GFN".
- s/__p2m_set_entry/_p2m_set_entry.
- drop "s" for sgfn and smfn prefixes of _p2m_set_entry()'s arguments
as this function work only with one GFN and one MFN.
- Return correct return code when p2m_next_level() faild in _p2m_set_entry(),
also drop "else" and just handle case (rc != P2M_TABLE_NORMAL) separately.
- Code style fixes.
- Use unsigned int for "order" in p2m_set_entry().
- s/p2m_set_entry/p2m_free_subtree.
- Update ASSERT() in __p2m_set_enty() to check that page_order is propertly
aligned.
- Return -EACCES instead of -ENOMEM in the chase when domain is dying and
someone called p2m_set_entry.
- s/p2m_set_entry/p2m_set_range.
- s/__p2m_set_entry/p2m_set_entry
- s/p2me_is_valid/p2m_is_valid()
- Return a number of successfully mapped GFNs in case if not all were mapped
in p2m_set_range().
- Use BIT(order, UL) instead of 1 << order.
- Drop IOMMU flushing code from p2m_set_entry().
- set p2m->need_flush=true when entry in p2m_set_entry() is changed.
- Introduce p2m_mapping_order() to support superpages.
- Drop p2m_is_valid() and use pte_is_valid() instead as there is no tricks
with copying of valid bit anymore.
- Update p2m_pte_from_mfn() prototype: drop p2m argument.
---
Changes in V2:
- New patch. It was a part of a big patch "xen/riscv: implement p2m mapping
functionality" which was splitted to smaller.
- Update the way when p2m TLB is flushed:
- RISC-V does't require BBM so there is no need to remove PTE before making
new so drop 'if /*pte_is_valid(orig_pte) */' and remove PTE only removing
has been requested.
- Drop p2m->need_flush |= !!pte_is_valid(orig_pte); for the case when
PTE's removing is happening as RISC-V could cache invalid PTE and thereby
it requires to do a flush each time and it doesn't matter if PTE is valid
or not at the moment when PTE removing is happening.
- Drop a check if PTE is valid in case of PTE is modified as it was mentioned
above as BBM isn't required so TLB flushing could be defered and there is
no need to do it before modifying of PTE.
- Drop p2m->need_flush as it seems like it will be always true.
- Drop foreign mapping things as it isn't necessary for RISC-V right now.
- s/p2m_is_valid/p2me_is_valid.
- Move definition and initalization of p2m->{max_mapped_gfn,lowest_mapped_gfn}
to this patch.
---
xen/arch/riscv/include/asm/p2m.h | 39 +++++
xen/arch/riscv/p2m.c | 281 ++++++++++++++++++++++++++++++-
2 files changed, 319 insertions(+), 1 deletion(-)
diff --git a/xen/arch/riscv/include/asm/p2m.h b/xen/arch/riscv/include/asm/p2m.h
index c98cf547f1..1a43736855 100644
--- a/xen/arch/riscv/include/asm/p2m.h
+++ b/xen/arch/riscv/include/asm/p2m.h
@@ -8,12 +8,41 @@
#include <xen/rwlock.h>
#include <xen/types.h>
+#include <asm/page.h>
#include <asm/page-bits.h>
extern unsigned long gstage_mode;
+extern unsigned int gstage_root_level;
#define P2M_ROOT_ORDER (ilog2(GSTAGE_ROOT_PAGE_TABLE_SIZE) - PAGE_SHIFT)
#define P2M_ROOT_PAGES BIT(P2M_ROOT_ORDER, U)
+#define P2M_ROOT_LEVEL gstage_root_level
+
+/*
+ * According to the RISC-V spec:
+ * When hgatp.MODE specifies a translation scheme of Sv32x4, Sv39x4, Sv48x4,
+ * or Sv57x4, G-stage address translation is a variation on the usual
+ * page-based virtual address translation scheme of Sv32, Sv39, Sv48, or
+ * Sv57, respectively. In each case, the size of the incoming address is
+ * widened by 2 bits (to 34, 41, 50, or 59 bits).
+ *
+ * P2M_LEVEL_ORDER(lvl) defines the bit position in the GFN from which
+ * the index for this level of the P2M page table starts. The extra 2
+ * bits added by the "x4" schemes only affect the root page table width.
+ *
+ * Therefore, this macro can safely reuse XEN_PT_LEVEL_ORDER() for all
+ * levels: the extra 2 bits do not change the indices of lower levels.
+ *
+ * The extra 2 bits are only relevant if one tried to address beyond the
+ * root level (i.e., P2M_LEVEL_ORDER(P2M_ROOT_LEVEL + 1)), which is
+ * invalid.
+ */
+#define P2M_LEVEL_ORDER(lvl) XEN_PT_LEVEL_ORDER(lvl)
+
+#define P2M_ROOT_EXTRA_BITS(lvl) (2 * ((lvl) == P2M_ROOT_LEVEL))
+
+#define P2M_PAGETABLE_ENTRIES(lvl) \
+ (BIT(PAGETABLE_ORDER + P2M_ROOT_EXTRA_BITS(lvl), UL))
#define paddr_bits PADDR_BITS
@@ -52,6 +81,16 @@ struct p2m_domain {
* when a page is needed to be fully cleared and cleaned.
*/
bool clean_dcache;
+
+ /* Highest guest frame that's ever been mapped in the p2m */
+ gfn_t max_mapped_gfn;
+
+ /*
+ * Lowest mapped gfn in the p2m. When releasing mapped gfn's in a
+ * preemptible manner this is updated to track where to resume
+ * the search. Apart from during teardown this can only decrease.
+ */
+ gfn_t lowest_mapped_gfn;
};
/*
diff --git a/xen/arch/riscv/p2m.c b/xen/arch/riscv/p2m.c
index d8b611961c..db9f7a77ff 100644
--- a/xen/arch/riscv/p2m.c
+++ b/xen/arch/riscv/p2m.c
@@ -16,6 +16,7 @@
#include <asm/riscv_encoding.h>
unsigned long __ro_after_init gstage_mode;
+unsigned int __ro_after_init gstage_root_level;
void __init gstage_mode_detect(void)
{
@@ -53,6 +54,7 @@ void __init gstage_mode_detect(void)
if ( MASK_EXTR(csr_read(CSR_HGATP), HGATP_MODE_MASK) == mode )
{
gstage_mode = mode;
+ gstage_root_level = modes[mode_idx].paging_levels - 1;
break;
}
}
@@ -210,6 +212,9 @@ int p2m_init(struct domain *d)
rwlock_init(&p2m->lock);
INIT_PAGE_LIST_HEAD(&p2m->pages);
+ p2m->max_mapped_gfn = _gfn(0);
+ p2m->lowest_mapped_gfn = _gfn(ULONG_MAX);
+
/*
* Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
* is not ready for RISC-V support.
@@ -251,13 +256,287 @@ int p2m_set_allocation(struct domain *d, unsigned long pages, bool *preempted)
return rc;
}
+/*
+ * Find and map the root page table. The caller is responsible for
+ * unmapping the table.
+ *
+ * The function will return NULL if the offset into the root table is
+ * invalid.
+ */
+static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
+{
+ unsigned long root_table_indx;
+
+ root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
+ if ( root_table_indx >= P2M_ROOT_PAGES )
+ return NULL;
+
+ /*
+ * The P2M root page table is extended by 2 bits, making its size 16KB
+ * (instead of 4KB for non-root page tables). Therefore, p2m->root is
+ * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
+ * only allocates 4KB pages).
+ *
+ * To determine which of these four 4KB pages the root_table_indx falls
+ * into, we divide root_table_indx by
+ * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
+ */
+ root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
+
+ return __map_domain_page(p2m->root + root_table_indx);
+}
+
+static inline void p2m_write_pte(pte_t *p, pte_t pte, bool clean_pte)
+{
+ write_pte(p, pte);
+
+ /*
+ * TODO: if multiple adjacent PTEs are written without releasing
+ * the lock, this then redundant cache flushing can be a
+ * performance issue.
+ */
+ if ( clean_pte )
+ clean_dcache_va_range(p, sizeof(*p));
+}
+
+static inline void p2m_clean_pte(pte_t *p, bool clean_pte)
+{
+ pte_t pte = { .pte = 0 };
+
+ p2m_write_pte(p, pte, clean_pte);
+}
+
+static pte_t p2m_pte_from_mfn(mfn_t mfn, p2m_type_t t)
+{
+ panic("%s: hasn't been implemented yet\n", __func__);
+
+ return (pte_t) { .pte = 0 };
+}
+
+#define P2M_TABLE_MAP_NONE 0
+#define P2M_TABLE_MAP_NOMEM 1
+#define P2M_TABLE_SUPER_PAGE 2
+#define P2M_TABLE_NORMAL 3
+
+/*
+ * Take the currently mapped table, find the entry corresponding to the GFN,
+ * and map the next-level table if available. The previous table will be
+ * unmapped if the next level was mapped (e.g., when P2M_TABLE_NORMAL is
+ * returned).
+ *
+ * `alloc_tbl` parameter indicates whether intermediate tables should
+ * be allocated when not present.
+ *
+ * Return values:
+ * P2M_TABLE_MAP_NONE: a table allocation isn't permitted.
+ * P2M_TABLE_MAP_NOMEM: allocating a new page failed.
+ * P2M_TABLE_SUPER_PAGE: next level or leaf mapped normally.
+ * P2M_TABLE_NORMAL: The next entry points to a superpage.
+ */
+static int p2m_next_level(struct p2m_domain *p2m, bool alloc_tbl,
+ unsigned int level, pte_t **table,
+ unsigned int offset)
+{
+ panic("%s: hasn't been implemented yet\n", __func__);
+
+ return P2M_TABLE_MAP_NONE;
+}
+
+/* Free pte sub-tree behind an entry */
+static void p2m_free_subtree(struct p2m_domain *p2m,
+ pte_t entry, unsigned int level)
+{
+ panic("%s: hasn't been implemented yet\n", __func__);
+}
+
+/*
+ * Insert an entry in the p2m. This should be called with a mapping
+ * equal to a page/superpage.
+ */
+static int p2m_set_entry(struct p2m_domain *p2m,
+ gfn_t gfn,
+ unsigned long page_order,
+ mfn_t mfn,
+ p2m_type_t t)
+{
+ unsigned int level;
+ unsigned int target = page_order / PAGETABLE_ORDER;
+ pte_t *entry, *table, orig_pte;
+ int rc;
+ /*
+ * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
+ * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
+ * are still allowed.
+ */
+ bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
+ DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
+
+ ASSERT(p2m_is_write_locked(p2m));
+
+ /*
+ * Check if the level target is valid: we only support
+ * 4K - 2M - 1G mapping.
+ */
+ ASSERT(target <= 2);
+
+ table = p2m_get_root_pointer(p2m, gfn);
+ if ( !table )
+ return -EINVAL;
+
+ for ( level = P2M_ROOT_LEVEL; level > target; level-- )
+ {
+ /*
+ * Don't try to allocate intermediate page table if the mapping
+ * is about to be removed.
+ */
+ rc = p2m_next_level(p2m, !removing_mapping,
+ level, &table, offsets[level]);
+ if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
+ {
+ rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
+ /*
+ * We are here because p2m_next_level has failed to map
+ * the intermediate page table (e.g the table does not exist
+ * and they p2m tree is read-only). It is a valid case
+ * when removing a mapping as it may not exist in the
+ * page table. In this case, just ignore it.
+ */
+ rc = removing_mapping ? 0 : rc;
+ goto out;
+ }
+
+ if ( rc != P2M_TABLE_NORMAL )
+ break;
+ }
+
+ entry = table + offsets[level];
+
+ /*
+ * If we are here with level > target, we must be at a leaf node,
+ * and we need to break up the superpage.
+ */
+ if ( level > target )
+ {
+ panic("Shattering isn't implemented\n");
+ }
+
+ /*
+ * We should always be there with the correct level because all the
+ * intermediate tables have been installed if necessary.
+ */
+ ASSERT(level == target);
+
+ orig_pte = *entry;
+
+ if ( removing_mapping )
+ p2m_clean_pte(entry, p2m->clean_dcache);
+ else
+ {
+ pte_t pte = p2m_pte_from_mfn(mfn, t);
+
+ p2m_write_pte(entry, pte, p2m->clean_dcache);
+
+ p2m->max_mapped_gfn = gfn_max(p2m->max_mapped_gfn,
+ gfn_add(gfn, BIT(page_order, UL) - 1));
+ p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, gfn);
+ }
+
+ p2m->need_flush = true;
+
+ /*
+ * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
+ * is not ready for RISC-V support.
+ *
+ * When CONFIG_HAS_PASSTHROUGH=y, iommu_iotlb_flush() should be done
+ * here.
+ */
+#ifdef CONFIG_HAS_PASSTHROUGH
+# error "add code to flush IOMMU TLB"
+#endif
+
+ rc = 0;
+
+ /*
+ * Free the entry only if the original pte was valid and the base
+ * is different (to avoid freeing when permission is changed).
+ *
+ * If previously MFN 0 was mapped and it is going to be removed
+ * and considering that during removing MFN 0 is used then `entry`
+ * and `new_entry` will be the same and p2m_free_subtree() won't be
+ * called. This case is handled explicitly.
+ */
+ if ( pte_is_valid(orig_pte) &&
+ (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
+ (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
+ p2m_free_subtree(p2m, orig_pte, level);
+
+ out:
+ unmap_domain_page(table);
+
+ return rc;
+}
+
+/* Return mapping order for given gfn, mfn and nr */
+static unsigned long p2m_mapping_order(gfn_t gfn, mfn_t mfn, unsigned long nr)
+{
+ unsigned long mask;
+ /* 1gb, 2mb, 4k mappings are supported */
+ unsigned int level = min(P2M_ROOT_LEVEL, _AC(2, U));
+ unsigned long order = 0;
+
+ mask = !mfn_eq(mfn, INVALID_MFN) ? mfn_x(mfn) : 0;
+ mask |= gfn_x(gfn);
+
+ for ( ; level != 0; level-- )
+ {
+ if ( !(mask & (BIT(P2M_LEVEL_ORDER(level), UL) - 1)) &&
+ (nr >= BIT(P2M_LEVEL_ORDER(level), UL)) )
+ {
+ order = P2M_LEVEL_ORDER(level);
+ break;
+ }
+ }
+
+ return order;
+}
+
static int p2m_set_range(struct p2m_domain *p2m,
gfn_t sgfn,
unsigned long nr,
mfn_t smfn,
p2m_type_t t)
{
- return -EOPNOTSUPP;
+ int rc = 0;
+ unsigned long left = nr;
+
+ /*
+ * Any reference taken by the P2M mappings (e.g. foreign mapping) will
+ * be dropped in relinquish_p2m_mapping(). As the P2M will still
+ * be accessible after, we need to prevent mapping to be added when the
+ * domain is dying.
+ */
+ if ( unlikely(p2m->domain->is_dying) )
+ return -EACCES;
+
+ while ( left )
+ {
+ unsigned long order = p2m_mapping_order(sgfn, smfn, left);
+
+ rc = p2m_set_entry(p2m, sgfn, order, smfn, t);
+ if ( rc )
+ break;
+
+ sgfn = gfn_add(sgfn, BIT(order, UL));
+ if ( !mfn_eq(smfn, INVALID_MFN) )
+ smfn = mfn_add(smfn, BIT(order, UL));
+
+ left -= BIT(order, UL);
+ }
+
+ if ( left > INT_MAX )
+ rc = -EOVERFLOW;
+
+ return !left ? rc : left;
}
int map_regions_p2mt(struct domain *d,
--
2.51.0
On 17.09.2025 23:55, Oleksii Kurochko wrote:
> --- a/xen/arch/riscv/p2m.c
> +++ b/xen/arch/riscv/p2m.c
> @@ -16,6 +16,7 @@
> #include <asm/riscv_encoding.h>
>
> unsigned long __ro_after_init gstage_mode;
> +unsigned int __ro_after_init gstage_root_level;
>
> void __init gstage_mode_detect(void)
> {
> @@ -53,6 +54,7 @@ void __init gstage_mode_detect(void)
> if ( MASK_EXTR(csr_read(CSR_HGATP), HGATP_MODE_MASK) == mode )
> {
> gstage_mode = mode;
> + gstage_root_level = modes[mode_idx].paging_levels - 1;
> break;
> }
> }
> @@ -210,6 +212,9 @@ int p2m_init(struct domain *d)
> rwlock_init(&p2m->lock);
> INIT_PAGE_LIST_HEAD(&p2m->pages);
>
> + p2m->max_mapped_gfn = _gfn(0);
> + p2m->lowest_mapped_gfn = _gfn(ULONG_MAX);
> +
> /*
> * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
> * is not ready for RISC-V support.
> @@ -251,13 +256,287 @@ int p2m_set_allocation(struct domain *d, unsigned long pages, bool *preempted)
> return rc;
> }
>
> +/*
> + * Find and map the root page table. The caller is responsible for
> + * unmapping the table.
With the root table being 4 pages, "the root table" is slightly misleading
here: Yu never map the entire table.
> + * The function will return NULL if the offset into the root table is
> + * invalid.
> + */
> +static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
> +{
> + unsigned long root_table_indx;
> +
> + root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
> + if ( root_table_indx >= P2M_ROOT_PAGES )
> + return NULL;
> +
> + /*
> + * The P2M root page table is extended by 2 bits, making its size 16KB
> + * (instead of 4KB for non-root page tables). Therefore, p2m->root is
> + * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
> + * only allocates 4KB pages).
> + *
> + * To determine which of these four 4KB pages the root_table_indx falls
> + * into, we divide root_table_indx by
> + * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
> + */
> + root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
The subtraction of 1 here feels odd: You're after the root table's
number of entries, i.e. I'd expect you to pass just P2M_ROOT_LEVEL.
And the way P2M_PAGETABLE_ENTRIES() works also suggests so.
> +/*
> + * Insert an entry in the p2m. This should be called with a mapping
> + * equal to a page/superpage.
> + */
I don't follow this comment: There isn't any mapping being passed in, is there?
> +static int p2m_set_entry(struct p2m_domain *p2m,
> + gfn_t gfn,
> + unsigned long page_order,
> + mfn_t mfn,
> + p2m_type_t t)
Nit: Indentation.
> +{
> + unsigned int level;
> + unsigned int target = page_order / PAGETABLE_ORDER;
> + pte_t *entry, *table, orig_pte;
> + int rc;
> + /*
> + * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
> + * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
> + * are still allowed.
> + */
> + bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
> + DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
> +
> + ASSERT(p2m_is_write_locked(p2m));
> +
> + /*
> + * Check if the level target is valid: we only support
> + * 4K - 2M - 1G mapping.
> + */
> + ASSERT(target <= 2);
> +
> + table = p2m_get_root_pointer(p2m, gfn);
> + if ( !table )
> + return -EINVAL;
> +
> + for ( level = P2M_ROOT_LEVEL; level > target; level-- )
> + {
> + /*
> + * Don't try to allocate intermediate page table if the mapping
> + * is about to be removed.
> + */
> + rc = p2m_next_level(p2m, !removing_mapping,
> + level, &table, offsets[level]);
> + if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
> + {
> + rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
> + /*
> + * We are here because p2m_next_level has failed to map
> + * the intermediate page table (e.g the table does not exist
> + * and they p2m tree is read-only).
I thought I commented on this or something similar already: Calling the
p2m tree "read-only" is imo misleading.
> It is a valid case
> + * when removing a mapping as it may not exist in the
> + * page table. In this case, just ignore it.
I fear the "it" has no reference; aiui you mean "ignore the lookup failure",
but the comment isn't worded to refer to that by "it".
> + */
> + rc = removing_mapping ? 0 : rc;
> + goto out;
> + }
> +
> + if ( rc != P2M_TABLE_NORMAL )
> + break;
> + }
> +
> + entry = table + offsets[level];
> +
> + /*
> + * If we are here with level > target, we must be at a leaf node,
> + * and we need to break up the superpage.
> + */
> + if ( level > target )
> + {
> + panic("Shattering isn't implemented\n");
> + }
> +
> + /*
> + * We should always be there with the correct level because all the
> + * intermediate tables have been installed if necessary.
> + */
> + ASSERT(level == target);
> +
> + orig_pte = *entry;
> +
> + if ( removing_mapping )
> + p2m_clean_pte(entry, p2m->clean_dcache);
> + else
> + {
> + pte_t pte = p2m_pte_from_mfn(mfn, t);
> +
> + p2m_write_pte(entry, pte, p2m->clean_dcache);
> +
> + p2m->max_mapped_gfn = gfn_max(p2m->max_mapped_gfn,
> + gfn_add(gfn, BIT(page_order, UL) - 1));
> + p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, gfn);
> + }
> +
> + p2m->need_flush = true;
> +
> + /*
> + * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
> + * is not ready for RISC-V support.
> + *
> + * When CONFIG_HAS_PASSTHROUGH=y, iommu_iotlb_flush() should be done
> + * here.
> + */
> +#ifdef CONFIG_HAS_PASSTHROUGH
> +# error "add code to flush IOMMU TLB"
> +#endif
> +
> + rc = 0;
> +
> + /*
> + * Free the entry only if the original pte was valid and the base
> + * is different (to avoid freeing when permission is changed).
> + *
> + * If previously MFN 0 was mapped and it is going to be removed
> + * and considering that during removing MFN 0 is used then `entry`
> + * and `new_entry` will be the same and p2m_free_subtree() won't be
> + * called. This case is handled explicitly.
> + */
> + if ( pte_is_valid(orig_pte) &&
> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
> + (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
> + p2m_free_subtree(p2m, orig_pte, level);
I continue to fail to understand why the MFN would matter here. Isn't the
need to free strictly tied to a VALID -> NOT VALID transition? A permission
change simply retains the VALID state of an entry.
Jan
On 9/20/25 1:36 AM, Jan Beulich wrote:
> On 17.09.2025 23:55, Oleksii Kurochko wrote:
>> --- a/xen/arch/riscv/p2m.c
>> +++ b/xen/arch/riscv/p2m.c
>> @@ -16,6 +16,7 @@
>> #include <asm/riscv_encoding.h>
>>
>> unsigned long __ro_after_init gstage_mode;
>> +unsigned int __ro_after_init gstage_root_level;
>>
>> void __init gstage_mode_detect(void)
>> {
>> @@ -53,6 +54,7 @@ void __init gstage_mode_detect(void)
>> if ( MASK_EXTR(csr_read(CSR_HGATP), HGATP_MODE_MASK) == mode )
>> {
>> gstage_mode = mode;
>> + gstage_root_level = modes[mode_idx].paging_levels - 1;
>> break;
>> }
>> }
>> @@ -210,6 +212,9 @@ int p2m_init(struct domain *d)
>> rwlock_init(&p2m->lock);
>> INIT_PAGE_LIST_HEAD(&p2m->pages);
>>
>> + p2m->max_mapped_gfn = _gfn(0);
>> + p2m->lowest_mapped_gfn = _gfn(ULONG_MAX);
>> +
>> /*
>> * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
>> * is not ready for RISC-V support.
>> @@ -251,13 +256,287 @@ int p2m_set_allocation(struct domain *d, unsigned long pages, bool *preempted)
>> return rc;
>> }
>>
>> +/*
>> + * Find and map the root page table. The caller is responsible for
>> + * unmapping the table.
> With the root table being 4 pages, "the root table" is slightly misleading
> here: Yu never map the entire table.
I will update the comment then to:
/*
* Map one of the four root pages of the P2M root page table.
*
* The P2M root page table is larger than normal (16KB instead of 4KB),
* so it is allocated as four consecutive 4KB pages. This function selects
* the appropriate 4KB page based on the given GFN and returns a mapping
* to it.
*
* The caller is responsible for unmapping the page after use.
*
* Returns NULL if the calculated offset into the root table is invalid.
*/
>
>> + * The function will return NULL if the offset into the root table is
>> + * invalid.
>> + */
>> +static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
>> +{
>> + unsigned long root_table_indx;
>> +
>> + root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
>> + if ( root_table_indx >= P2M_ROOT_PAGES )
>> + return NULL;
>> +
>> + /*
>> + * The P2M root page table is extended by 2 bits, making its size 16KB
>> + * (instead of 4KB for non-root page tables). Therefore, p2m->root is
>> + * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
>> + * only allocates 4KB pages).
>> + *
>> + * To determine which of these four 4KB pages the root_table_indx falls
>> + * into, we divide root_table_indx by
>> + * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
>> + */
>> + root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
> The subtraction of 1 here feels odd: You're after the root table's
> number of entries, i.e. I'd expect you to pass just P2M_ROOT_LEVEL.
> And the way P2M_PAGETABLE_ENTRIES() works also suggests so.
The purpose of this line is to select the page within the root table, which
consists of 4 consecutive pages. However, P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL)
returns 2048, so root_table_idx will always be 0 after devision, which is not
what we want.
As an alternative, P2M_PAGETABLE_ENTRIES(0) could be used, since it always
returns 512. Dividing root_table_idx by 512 then yields the index of the page
within the root table, which is made up of 4 consecutive pages.
Does it make sense now?
The problem may occur with DECLARE_OFFSET(), which can produce an incorrect
index within the root page table. Since the index is in the range [0, 2047],
it becomes an issue if the value is greater than 511, because DECLARE_OFFSET()
does not account for the larger range of the root index.
I am not sure whether it is better to make DECLARE_OFFSET() generic enough
for both P2M and Xen page tables, or to provide a separate P2M_DECLARE_OFFSET()
and use it only in P2M-related code.
Also, it could be an option to move DECLARE_OFFSET() from asm/page.h header
to riscv/pt.c and define another one DECLARE_OFFSETS in riscv/p2m.c.
Do you have a preference?
>
>> +/*
>> + * Insert an entry in the p2m. This should be called with a mapping
>> + * equal to a page/superpage.
>> + */
> I don't follow this comment: There isn't any mapping being passed in, is there?
I think this comment should be dropped, it was about that requested mapping
should be equal to a page/superpage(4k, 2m, 1g), the correct order is always
guaranteed by p2m_mapping_order().
>
>> +static int p2m_set_entry(struct p2m_domain *p2m,
>> + gfn_t gfn,
>> + unsigned long page_order,
>> + mfn_t mfn,
>> + p2m_type_t t)
> Nit: Indentation.
>
>> +{
>> + unsigned int level;
>> + unsigned int target = page_order / PAGETABLE_ORDER;
>> + pte_t *entry, *table, orig_pte;
>> + int rc;
>> + /*
>> + * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
>> + * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
>> + * are still allowed.
>> + */
>> + bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
>> + DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
>> +
>> + ASSERT(p2m_is_write_locked(p2m));
>> +
>> + /*
>> + * Check if the level target is valid: we only support
>> + * 4K - 2M - 1G mapping.
>> + */
>> + ASSERT(target <= 2);
>> +
>> + table = p2m_get_root_pointer(p2m, gfn);
>> + if ( !table )
>> + return -EINVAL;
>> +
>> + for ( level = P2M_ROOT_LEVEL; level > target; level-- )
>> + {
>> + /*
>> + * Don't try to allocate intermediate page table if the mapping
>> + * is about to be removed.
>> + */
>> + rc = p2m_next_level(p2m, !removing_mapping,
>> + level, &table, offsets[level]);
>> + if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
>> + {
>> + rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
>> + /*
>> + * We are here because p2m_next_level has failed to map
>> + * the intermediate page table (e.g the table does not exist
>> + * and they p2m tree is read-only).
> I thought I commented on this or something similar already: Calling the
> p2m tree "read-only" is imo misleading.
I will change then "read-only" to "not allocatable".
>
>> It is a valid case
>> + * when removing a mapping as it may not exist in the
>> + * page table. In this case, just ignore it.
> I fear the "it" has no reference; aiui you mean "ignore the lookup failure",
> but the comment isn't worded to refer to that by "it".
I will update the comment correspondingly.
>
>> + */
>> + rc = removing_mapping ? 0 : rc;
>> + goto out;
>> + }
>> +
>> + if ( rc != P2M_TABLE_NORMAL )
>> + break;
>> + }
>> +
>> + entry = table + offsets[level];
>> +
>> + /*
>> + * If we are here with level > target, we must be at a leaf node,
>> + * and we need to break up the superpage.
>> + */
>> + if ( level > target )
>> + {
>> + panic("Shattering isn't implemented\n");
>> + }
>> +
>> + /*
>> + * We should always be there with the correct level because all the
>> + * intermediate tables have been installed if necessary.
>> + */
>> + ASSERT(level == target);
>> +
>> + orig_pte = *entry;
>> +
>> + if ( removing_mapping )
>> + p2m_clean_pte(entry, p2m->clean_dcache);
>> + else
>> + {
>> + pte_t pte = p2m_pte_from_mfn(mfn, t);
>> +
>> + p2m_write_pte(entry, pte, p2m->clean_dcache);
>> +
>> + p2m->max_mapped_gfn = gfn_max(p2m->max_mapped_gfn,
>> + gfn_add(gfn, BIT(page_order, UL) - 1));
>> + p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, gfn);
>> + }
>> +
>> + p2m->need_flush = true;
>> +
>> + /*
>> + * Currently, the infrastructure required to enable CONFIG_HAS_PASSTHROUGH
>> + * is not ready for RISC-V support.
>> + *
>> + * When CONFIG_HAS_PASSTHROUGH=y, iommu_iotlb_flush() should be done
>> + * here.
>> + */
>> +#ifdef CONFIG_HAS_PASSTHROUGH
>> +# error "add code to flush IOMMU TLB"
>> +#endif
>> +
>> + rc = 0;
>> +
>> + /*
>> + * Free the entry only if the original pte was valid and the base
>> + * is different (to avoid freeing when permission is changed).
>> + *
>> + * If previously MFN 0 was mapped and it is going to be removed
>> + * and considering that during removing MFN 0 is used then `entry`
>> + * and `new_entry` will be the same and p2m_free_subtree() won't be
>> + * called. This case is handled explicitly.
>> + */
>> + if ( pte_is_valid(orig_pte) &&
>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
>> + (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
>> + p2m_free_subtree(p2m, orig_pte, level);
> I continue to fail to understand why the MFN would matter here.
My understanding is that if, for the same GFN, the MFN changes fromMFN_1 to
MFN_2, then we need to update any references on the page referenced by
|orig_pte| to ensure the proper reference counter is maintained for the page
pointed to byMFN_1.
> Isn't the
> need to free strictly tied to a VALID -> NOT VALID transition? A permission
> change simply retains the VALID state of an entry.
It covers a case when removing happens and probably in this case we don't need
to check specifically for mfn(0) case "mfn_eq(pte_get_mfn(*entry), _mfn(0))",
but it would be enough to check that pte_is_valid(entry) instead:
...
(removing_mapping && !pte_is_valid(entry)))) )
Or only check removing_mapping variable as `entry` would be invalided by the
code above anyway. So we will get:
+ if ( pte_is_valid(orig_pte) &&
+ (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) || removing_mapping) )
+ p2m_free_subtree(p2m, orig_pte, level);
Does it make sense now?
~ Oleksii
On 25.09.2025 22:08, Oleksii Kurochko wrote:
> On 9/20/25 1:36 AM, Jan Beulich wrote:
>> On 17.09.2025 23:55, Oleksii Kurochko wrote:
>>> +static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
>>> +{
>>> + unsigned long root_table_indx;
>>> +
>>> + root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
>>> + if ( root_table_indx >= P2M_ROOT_PAGES )
>>> + return NULL;
>>> +
>>> + /*
>>> + * The P2M root page table is extended by 2 bits, making its size 16KB
>>> + * (instead of 4KB for non-root page tables). Therefore, p2m->root is
>>> + * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
>>> + * only allocates 4KB pages).
>>> + *
>>> + * To determine which of these four 4KB pages the root_table_indx falls
>>> + * into, we divide root_table_indx by
>>> + * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
>>> + */
>>> + root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
>> The subtraction of 1 here feels odd: You're after the root table's
>> number of entries, i.e. I'd expect you to pass just P2M_ROOT_LEVEL.
>> And the way P2M_PAGETABLE_ENTRIES() works also suggests so.
>
> The purpose of this line is to select the page within the root table, which
> consists of 4 consecutive pages. However, P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL)
> returns 2048, so root_table_idx will always be 0 after devision, which is not
> what we want.
>
> As an alternative, P2M_PAGETABLE_ENTRIES(0) could be used, since it always
> returns 512. Dividing root_table_idx by 512 then yields the index of the page
> within the root table, which is made up of 4 consecutive pages.
>
> Does it make sense now?
Yes and no. I understand what you're after, but that doesn't make the use of
P2M_PAGETABLE_ENTRIES() (with an arbitrary level as argument) correct. This
calculation wants doing by solely using properties of the top level.
> The problem may occur with DECLARE_OFFSET(), which can produce an incorrect
> index within the root page table. Since the index is in the range [0, 2047],
> it becomes an issue if the value is greater than 511, because DECLARE_OFFSET()
> does not account for the larger range of the root index.
>
> I am not sure whether it is better to make DECLARE_OFFSET() generic enough
> for both P2M and Xen page tables, or to provide a separate P2M_DECLARE_OFFSET()
> and use it only in P2M-related code.
> Also, it could be an option to move DECLARE_OFFSET() from asm/page.h header
> to riscv/pt.c and define another one DECLARE_OFFSETS in riscv/p2m.c.
>
> Do you have a preference?
Not really, no. I don't like DECLARE_OFFSETS() anyway.
>>> +static int p2m_set_entry(struct p2m_domain *p2m,
>>> + gfn_t gfn,
>>> + unsigned long page_order,
>>> + mfn_t mfn,
>>> + p2m_type_t t)
>> Nit: Indentation.
>>
>>> +{
>>> + unsigned int level;
>>> + unsigned int target = page_order / PAGETABLE_ORDER;
>>> + pte_t *entry, *table, orig_pte;
>>> + int rc;
>>> + /*
>>> + * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
>>> + * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
>>> + * are still allowed.
>>> + */
>>> + bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
>>> + DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
>>> +
>>> + ASSERT(p2m_is_write_locked(p2m));
>>> +
>>> + /*
>>> + * Check if the level target is valid: we only support
>>> + * 4K - 2M - 1G mapping.
>>> + */
>>> + ASSERT(target <= 2);
>>> +
>>> + table = p2m_get_root_pointer(p2m, gfn);
>>> + if ( !table )
>>> + return -EINVAL;
>>> +
>>> + for ( level = P2M_ROOT_LEVEL; level > target; level-- )
>>> + {
>>> + /*
>>> + * Don't try to allocate intermediate page table if the mapping
>>> + * is about to be removed.
>>> + */
>>> + rc = p2m_next_level(p2m, !removing_mapping,
>>> + level, &table, offsets[level]);
>>> + if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
>>> + {
>>> + rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
>>> + /*
>>> + * We are here because p2m_next_level has failed to map
>>> + * the intermediate page table (e.g the table does not exist
>>> + * and they p2m tree is read-only).
>> I thought I commented on this or something similar already: Calling the
>> p2m tree "read-only" is imo misleading.
>
> I will change then "read-only" to "not allocatable".
That'll be only marginally better: What's "allocatable"? Why not something
like "... does not exist and none should be allocated"? Or maybe simply
omit this part of the comment?
>>> + /*
>>> + * Free the entry only if the original pte was valid and the base
>>> + * is different (to avoid freeing when permission is changed).
>>> + *
>>> + * If previously MFN 0 was mapped and it is going to be removed
>>> + * and considering that during removing MFN 0 is used then `entry`
>>> + * and `new_entry` will be the same and p2m_free_subtree() won't be
>>> + * called. This case is handled explicitly.
>>> + */
>>> + if ( pte_is_valid(orig_pte) &&
>>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
>>> + (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
>>> + p2m_free_subtree(p2m, orig_pte, level);
>> I continue to fail to understand why the MFN would matter here.
>
> My understanding is that if, for the same GFN, the MFN changes fromMFN_1 to
> MFN_2, then we need to update any references on the page referenced by
> |orig_pte| to ensure the proper reference counter is maintained for the page
> pointed to byMFN_1.
>
>> Isn't the
>> need to free strictly tied to a VALID -> NOT VALID transition? A permission
>> change simply retains the VALID state of an entry.
>
> It covers a case when removing happens and probably in this case we don't need
> to check specifically for mfn(0) case "mfn_eq(pte_get_mfn(*entry), _mfn(0))",
> but it would be enough to check that pte_is_valid(entry) instead:
> ...
> (removing_mapping && !pte_is_valid(entry)))) )
>
> Or only check removing_mapping variable as `entry` would be invalided by the
> code above anyway. So we will get:
> + if ( pte_is_valid(orig_pte) &&
> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) || removing_mapping) )
> + p2m_free_subtree(p2m, orig_pte, level);
>
> Does it make sense now?
Not really, sorry. Imo the complicated condition indicates that something is
wrong (or at least inefficient) here.
Jan
On 9/26/25 9:07 AM, Jan Beulich wrote:
> On 25.09.2025 22:08, Oleksii Kurochko wrote:
>> On 9/20/25 1:36 AM, Jan Beulich wrote:
>>> On 17.09.2025 23:55, Oleksii Kurochko wrote:
>>>> +static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
>>>> +{
>>>> + unsigned long root_table_indx;
>>>> +
>>>> + root_table_indx = gfn_x(gfn) >> P2M_LEVEL_ORDER(P2M_ROOT_LEVEL);
>>>> + if ( root_table_indx >= P2M_ROOT_PAGES )
>>>> + return NULL;
>>>> +
>>>> + /*
>>>> + * The P2M root page table is extended by 2 bits, making its size 16KB
>>>> + * (instead of 4KB for non-root page tables). Therefore, p2m->root is
>>>> + * allocated as four consecutive 4KB pages (since alloc_domheap_pages()
>>>> + * only allocates 4KB pages).
>>>> + *
>>>> + * To determine which of these four 4KB pages the root_table_indx falls
>>>> + * into, we divide root_table_indx by
>>>> + * P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1).
>>>> + */
>>>> + root_table_indx /= P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL - 1);
>>> The subtraction of 1 here feels odd: You're after the root table's
>>> number of entries, i.e. I'd expect you to pass just P2M_ROOT_LEVEL.
>>> And the way P2M_PAGETABLE_ENTRIES() works also suggests so.
>> The purpose of this line is to select the page within the root table, which
>> consists of 4 consecutive pages. However, P2M_PAGETABLE_ENTRIES(P2M_ROOT_LEVEL)
>> returns 2048, so root_table_idx will always be 0 after devision, which is not
>> what we want.
>>
>> As an alternative, P2M_PAGETABLE_ENTRIES(0) could be used, since it always
>> returns 512. Dividing root_table_idx by 512 then yields the index of the page
>> within the root table, which is made up of 4 consecutive pages.
>>
>> Does it make sense now?
> Yes and no. I understand what you're after, but that doesn't make the use of
> P2M_PAGETABLE_ENTRIES() (with an arbitrary level as argument) correct. This
> calculation wants doing by solely using properties of the top level.
Got it, thanks. Then I will use solely properties of the top level.
>>>> +static int p2m_set_entry(struct p2m_domain *p2m,
>>>> + gfn_t gfn,
>>>> + unsigned long page_order,
>>>> + mfn_t mfn,
>>>> + p2m_type_t t)
>>> Nit: Indentation.
>>>
>>>> +{
>>>> + unsigned int level;
>>>> + unsigned int target = page_order / PAGETABLE_ORDER;
>>>> + pte_t *entry, *table, orig_pte;
>>>> + int rc;
>>>> + /*
>>>> + * A mapping is removed only if the MFN is explicitly set to INVALID_MFN.
>>>> + * Other MFNs that are considered invalid by mfn_valid() (e.g., MMIO)
>>>> + * are still allowed.
>>>> + */
>>>> + bool removing_mapping = mfn_eq(mfn, INVALID_MFN);
>>>> + DECLARE_OFFSETS(offsets, gfn_to_gaddr(gfn));
>>>> +
>>>> + ASSERT(p2m_is_write_locked(p2m));
>>>> +
>>>> + /*
>>>> + * Check if the level target is valid: we only support
>>>> + * 4K - 2M - 1G mapping.
>>>> + */
>>>> + ASSERT(target <= 2);
>>>> +
>>>> + table = p2m_get_root_pointer(p2m, gfn);
>>>> + if ( !table )
>>>> + return -EINVAL;
>>>> +
>>>> + for ( level = P2M_ROOT_LEVEL; level > target; level-- )
>>>> + {
>>>> + /*
>>>> + * Don't try to allocate intermediate page table if the mapping
>>>> + * is about to be removed.
>>>> + */
>>>> + rc = p2m_next_level(p2m, !removing_mapping,
>>>> + level, &table, offsets[level]);
>>>> + if ( (rc == P2M_TABLE_MAP_NONE) || (rc == P2M_TABLE_MAP_NOMEM) )
>>>> + {
>>>> + rc = (rc == P2M_TABLE_MAP_NONE) ? -ENOENT : -ENOMEM;
>>>> + /*
>>>> + * We are here because p2m_next_level has failed to map
>>>> + * the intermediate page table (e.g the table does not exist
>>>> + * and they p2m tree is read-only).
>>> I thought I commented on this or something similar already: Calling the
>>> p2m tree "read-only" is imo misleading.
>> I will change then "read-only" to "not allocatable".
> That'll be only marginally better: What's "allocatable"? Why not something
> like "... does not exist and none should be allocated"? Or maybe simply
> omit this part of the comment?
Agree, "allocatable" could be also confusing. Perhaps, just omitting will
be fine.
>
>>>> + /*
>>>> + * Free the entry only if the original pte was valid and the base
>>>> + * is different (to avoid freeing when permission is changed).
>>>> + *
>>>> + * If previously MFN 0 was mapped and it is going to be removed
>>>> + * and considering that during removing MFN 0 is used then `entry`
>>>> + * and `new_entry` will be the same and p2m_free_subtree() won't be
>>>> + * called. This case is handled explicitly.
>>>> + */
>>>> + if ( pte_is_valid(orig_pte) &&
>>>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
>>>> + (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
>>>> + p2m_free_subtree(p2m, orig_pte, level);
>>> I continue to fail to understand why the MFN would matter here.
>> My understanding is that if, for the same GFN, the MFN changes fromMFN_1 to
>> MFN_2, then we need to update any references on the page referenced by
>> |orig_pte| to ensure the proper reference counter is maintained for the page
>> pointed to byMFN_1.
>>
>>> Isn't the
>>> need to free strictly tied to a VALID -> NOT VALID transition? A permission
>>> change simply retains the VALID state of an entry.
>> It covers a case when removing happens and probably in this case we don't need
>> to check specifically for mfn(0) case "mfn_eq(pte_get_mfn(*entry), _mfn(0))",
>> but it would be enough to check that pte_is_valid(entry) instead:
>> ...
>> (removing_mapping && !pte_is_valid(entry)))) )
>>
>> Or only check removing_mapping variable as `entry` would be invalided by the
>> code above anyway. So we will get:
>> + if ( pte_is_valid(orig_pte) &&
>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) || removing_mapping) )
>> + p2m_free_subtree(p2m, orig_pte, level);
>>
>> Does it make sense now?
> Not really, sorry. Imo the complicated condition indicates that something is
> wrong (or at least inefficient) here.
Then, in the case of aVALID -> VALID transition, where the MFN is changed for the
same PTE, should something be done with the old MFN (e.g., calling|p2m_put_page()|
for it), or can freeing the old MFN be delayed until|domain_relinquish_resources() |is called? If so, wouldn’t that lead to a situation where many old MFNs accumulate
and cannot be re-used until|domain_relinquish_resources()| (or another function that
explicitly frees pages) is invoked?
If we only need to care about theVALID -> NOT VALID transition, doesn’t that mean
|p2m_free_subtree()| should be called only when a removal actually occurs?
~ Oleksii
On 9/26/25 10:58 AM, Oleksii Kurochko wrote:
>>>>> + /*
>>>>> + * Free the entry only if the original pte was valid and the base
>>>>> + * is different (to avoid freeing when permission is changed).
>>>>> + *
>>>>> + * If previously MFN 0 was mapped and it is going to be removed
>>>>> + * and considering that during removing MFN 0 is used then `entry`
>>>>> + * and `new_entry` will be the same and p2m_free_subtree() won't be
>>>>> + * called. This case is handled explicitly.
>>>>> + */
>>>>> + if ( pte_is_valid(orig_pte) &&
>>>>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) ||
>>>>> + (removing_mapping && mfn_eq(pte_get_mfn(*entry), _mfn(0)))) )
>>>>> + p2m_free_subtree(p2m, orig_pte, level);
>>>> I continue to fail to understand why the MFN would matter here.
>>> My understanding is that if, for the same GFN, the MFN changes fromMFN_1 to
>>> MFN_2, then we need to update any references on the page referenced by
>>> |orig_pte| to ensure the proper reference counter is maintained for the page
>>> pointed to byMFN_1.
>>>
>>>> Isn't the
>>>> need to free strictly tied to a VALID -> NOT VALID transition? A permission
>>>> change simply retains the VALID state of an entry.
>>> It covers a case when removing happens and probably in this case we don't need
>>> to check specifically for mfn(0) case "mfn_eq(pte_get_mfn(*entry), _mfn(0))",
>>> but it would be enough to check that pte_is_valid(entry) instead:
>>> ...
>>> (removing_mapping && !pte_is_valid(entry)))) )
>>>
>>> Or only check removing_mapping variable as `entry` would be invalided by the
>>> code above anyway. So we will get:
>>> + if ( pte_is_valid(orig_pte) &&
>>> + (!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte)) || removing_mapping) )
>>> + p2m_free_subtree(p2m, orig_pte, level);
>>>
>>> Does it make sense now?
>> Not really, sorry. Imo the complicated condition indicates that something is
>> wrong (or at least inefficient) here.
> Then, in the case of aVALID -> VALID transition, where the MFN is changed for the
> same PTE, should something be done with the old MFN (e.g., calling|p2m_put_page()|
> for it), or can freeing the old MFN be delayed until|domain_relinquish_resources() |is called? If so, wouldn’t that lead to a situation where many old MFNs accumulate
> and cannot be re-used until|domain_relinquish_resources()| (or another function that
> explicitly frees pages) is invoked?
> If we only need to care about theVALID -> NOT VALID transition, doesn’t that mean
> |p2m_free_subtree()| should be called only when a removal actually occurs?
I've decided to "simplify" the original condition to:
/*
* In case of a VALID -> INVALID transition, the original PTE should
* always be freed.
*
* In case of a VALID -> VALID transition, the original PTE should be
* freed only if the MFNs are different. If the MFNs are the same
* (i.e., only permissions differ), there is no need to free the
* original PTE.
*/
if ( pte_is_valid(orig_pte) &&
(!pte_is_valid(*entry) ||
!mfn_eq(pte_get_mfn(*entry), pte_get_mfn(orig_pte))) )
{
I hope it would make more sense.
~ Oleksii
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