From: Penny Zheng <Penny.Zheng@arm.com>

We are doing foreign memory mapping for static shared memory, and
there is a great possibility that it could be super mapped.
But today, p2m_put_l3_page could not handle superpages.

This commits implements a new function p2m_put_l2_superpage to handle
level 2 superpages, specifically for helping put extra references for
foreign superpages.

Modify relinquish_p2m_mapping as well to take into account preemption
when we have a level-2 foreign mapping.

Currently level 1 superpages are not handled because Xen is not
preemptible and therefore some work is needed to handle such superpages,
for which at some point Xen might end up freeing memory and therefore
for such a big mapping it could end up in a very long operation.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
---
v4.2 changes:
 - rework commit message to don't explicit say the size of the mapping
   but only say the level, remove unnecessary p2m_is_superpage() check
   in p2m_put_page, remove (level >= 2) condition to call p2m_put_page
   inside p2m_free_entry because the code in that function can already
   handle the levels, move TODO comment inside p2m_put_page, avoid
   mentioning 2MB in the comments. (Julien)
 - This patch is meant to superseed the homonymous patch in the serie:
   https://patchwork.kernel.org/project/xen-devel/cover/20240524124055.3871399-1-luca.fancellu@arm.com/
v4 changes:
 - optimised the path to call put_page() on the foreign mapping as
   Julien suggested. Add a comment in p2m_put_l2_superpage to state
   that any changes needs to take into account some change in the
   relinquish code. (Julien)
v3 changes:
 - Add reasoning why we don't support now 1GB superpage, remove level_order
   variable from p2m_put_l2_superpage, update TODO comment inside
   p2m_free_entry, use XEN_PT_LEVEL_ORDER(2) instead of value 9 inside
   relinquish_p2m_mapping. (Michal)
v2:
 - Do not handle 1GB super page as there might be some issue where
   a lot of calls to put_page(...) might be issued which could lead
   to free memory that is a long operation.
v1:
 - patch from https://patchwork.kernel.org/project/xen-devel/patch/20231206090623.1932275-9-Penny.Zheng@arm.com/
---
 xen/arch/arm/mmu/p2m.c | 81 +++++++++++++++++++++++++++++++-----------
 1 file changed, 61 insertions(+), 20 deletions(-)

diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
index 41fcca011cf4..1725cca649b5 100644
--- a/xen/arch/arm/mmu/p2m.c
+++ b/xen/arch/arm/mmu/p2m.c
@@ -753,34 +753,72 @@ static int p2m_mem_access_radix_set(struct p2m_domain *p2m, gfn_t gfn,
     return rc;
 }
 
-/*
- * Put any references on the single 4K page referenced by pte.
- * TODO: Handle superpages, for now we only take special references for leaf
- * pages (specifically foreign ones, which can't be super mapped today).
- */
-static void p2m_put_l3_page(const lpae_t pte)
+static void p2m_put_foreign_page(struct page_info *pg)
 {
-    mfn_t mfn = lpae_get_mfn(pte);
-
-    ASSERT(p2m_is_valid(pte));
-
     /*
-     * TODO: Handle other p2m types
-     *
      * It's safe to do the put_page here because page_alloc will
      * flush the TLBs if the page is reallocated before the end of
      * this loop.
      */
-    if ( p2m_is_foreign(pte.p2m.type) )
+    put_page(pg);
+}
+
+/* Put any references on the single 4K page referenced by mfn. */
+static void p2m_put_l3_page(mfn_t mfn, p2m_type_t type)
+{
+    /* TODO: Handle other p2m types */
+    if ( p2m_is_foreign(type) )
     {
         ASSERT(mfn_valid(mfn));
-        put_page(mfn_to_page(mfn));
+        p2m_put_foreign_page(mfn_to_page(mfn));
     }
     /* Detect the xenheap page and mark the stored GFN as invalid. */
-    else if ( p2m_is_ram(pte.p2m.type) && is_xen_heap_mfn(mfn) )
+    else if ( p2m_is_ram(type) && is_xen_heap_mfn(mfn) )
         page_set_xenheap_gfn(mfn_to_page(mfn), INVALID_GFN);
 }
 
+/* Put any references on the superpage referenced by mfn. */
+static void p2m_put_l2_superpage(mfn_t mfn, p2m_type_t type)
+{
+    struct page_info *pg;
+    unsigned int i;
+
+    /*
+     * TODO: Handle other p2m types, but be aware that any changes to handle
+     * different types should require an update on the relinquish code to handle
+     * preemption.
+     */
+    if ( !p2m_is_foreign(type) )
+        return;
+
+    ASSERT(mfn_valid(mfn));
+
+    pg = mfn_to_page(mfn);
+
+    for ( i = 0; i < XEN_PT_LPAE_ENTRIES; i++, pg++ )
+        p2m_put_foreign_page(pg);
+}
+
+/* Put any references on the page referenced by pte. */
+static void p2m_put_page(const lpae_t pte, unsigned int level)
+{
+    mfn_t mfn = lpae_get_mfn(pte);
+
+    ASSERT(p2m_is_valid(pte));
+
+    /*
+     * TODO: Currently we don't handle level 1 super-page, Xen is not
+     * preemptible and therefore some work is needed to handle such
+     * superpages, for which at some point Xen might end up freeing memory
+     * and therefore for such a big mapping it could end up in a very long
+     * operation.
+     */
+    if ( level == 2 )
+        return p2m_put_l2_superpage(mfn, pte.p2m.type);
+    else if ( level == 3 )
+        return p2m_put_l3_page(mfn, pte.p2m.type);
+}
+
 /* Free lpae sub-tree behind an entry */
 static void p2m_free_entry(struct p2m_domain *p2m,
                            lpae_t entry, unsigned int level)
@@ -809,9 +847,9 @@ static void p2m_free_entry(struct p2m_domain *p2m,
 #endif
 
         p2m->stats.mappings[level]--;
-        /* Nothing to do if the entry is a super-page. */
-        if ( level == 3 )
-            p2m_put_l3_page(entry);
+
+        p2m_put_page(entry, level);
+
         return;
     }
 
@@ -1558,9 +1596,12 @@ int relinquish_p2m_mapping(struct domain *d)
 
         count++;
         /*
-         * Arbitrarily preempt every 512 iterations.
+         * Arbitrarily preempt every 512 iterations or when we have a level-2
+         * foreign mapping.
          */
-        if ( !(count % 512) && hypercall_preempt_check() )
+        if ( (!(count % 512) ||
+              (p2m_is_foreign(t) && (order > XEN_PT_LEVEL_ORDER(2)))) &&
+             hypercall_preempt_check() )
         {
             rc = -ERESTART;
             break;
-- 
2.34.1

Hi Luca,

On 28/05/2024 13:56, Luca Fancellu wrote:
> From: Penny Zheng <Penny.Zheng@arm.com>
> 
> We are doing foreign memory mapping for static shared memory, and
> there is a great possibility that it could be super mapped.
> But today, p2m_put_l3_page could not handle superpages.
> 
> This commits implements a new function p2m_put_l2_superpage to handle
> level 2 superpages, specifically for helping put extra references for
> foreign superpages.
> 
> Modify relinquish_p2m_mapping as well to take into account preemption
> when we have a level-2 foreign mapping.
> 
> Currently level 1 superpages are not handled because Xen is not
> preemptible and therefore some work is needed to handle such superpages,
> for which at some point Xen might end up freeing memory and therefore
> for such a big mapping it could end up in a very long operation.
> 
> Signed-off-by: Penny Zheng <penny.zheng@arm.com>
> Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>

Reviewed-by: Julien Grall <jgrall@amazon.com>

Cheers,

-- 
Julien Grall