Iterate over the p2m up to the maximum recorded gfn and remove any foreign
mappings, in order to drop the underlying page references and thus don't keep
extra page references if a domain is destroyed while still having foreign
mappings on it's p2m.

The logic is similar to the one used on Arm.

Note that foreign mappings cannot be created by guests that have altp2m or
nested HVM enabled, as p2ms different than the host one are not currently
scrubbed when destroyed in order to drop references to any foreign maps.

It's unclear whether the right solution is to take an extra reference when
foreign maps are added to p2ms different than the host one, or just rely on the
host p2m already having a reference.  The mapping being removed from the host
p2m should cause it to be dropped on all domain p2ms.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
Changes since v2:
 - Fix arch_acquire_resource_check() condition.

Changes since v1:
 - Use existing p2m max_mapped_pfn field.
 - Prevent creating foreign mappings by guests that have altp2m or nestedhvm
   enabled.
---
 CHANGELOG.md                   |  1 +
 xen/arch/x86/domain.c          |  6 +++
 xen/arch/x86/include/asm/p2m.h | 26 +++++++------
 xen/arch/x86/mm/p2m-basic.c    | 18 +++++++++
 xen/arch/x86/mm/p2m.c          | 68 ++++++++++++++++++++++++++++++++--
 5 files changed, 103 insertions(+), 16 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c43c45d8d4bd..59aad04e9364 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - xl/libxl configures vkb=[] for HVM domains with priority over vkb_device.
  - Increase the maximum number of CPUs Xen can be built for from 4095 to
    16383.
+ - Allow HVM/PVH domains to map foreign pages.
 
 ### Added
  - On x86:
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index a89ceff9ae93..de5f6b3212fb 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2386,6 +2386,7 @@ int domain_relinquish_resources(struct domain *d)
         enum {
             PROG_iommu_pagetables = 1,
             PROG_shared,
+            PROG_mappings,
             PROG_paging,
             PROG_vcpu_pagetables,
             PROG_xen,
@@ -2434,6 +2435,11 @@ int domain_relinquish_resources(struct domain *d)
         }
 #endif
 
+    PROGRESS(mappings):
+        ret = relinquish_p2m_mapping(d);
+        if ( ret )
+            return ret;
+
     PROGRESS(paging):
 
         /* Tear down paging-assistance stuff. */
diff --git a/xen/arch/x86/include/asm/p2m.h b/xen/arch/x86/include/asm/p2m.h
index 107b9f260848..c1478ffc3647 100644
--- a/xen/arch/x86/include/asm/p2m.h
+++ b/xen/arch/x86/include/asm/p2m.h
@@ -383,6 +383,8 @@ struct p2m_domain {
 
     /* Number of foreign mappings. */
     unsigned long      nr_foreign;
+    /* Cursor for iterating over the p2m on teardown. */
+    unsigned long      teardown_gfn;
 #endif /* CONFIG_HVM */
 };
 
@@ -395,16 +397,7 @@ struct p2m_domain {
 #endif
 #include <xen/p2m-common.h>
 
-static inline bool arch_acquire_resource_check(struct domain *d)
-{
-    /*
-     * FIXME: Until foreign pages inserted into the P2M are properly
-     * reference counted, it is unsafe to allow mapping of
-     * resource pages unless the caller is the hardware domain
-     * (see set_foreign_p2m_entry()).
-     */
-    return !paging_mode_translate(d) || is_hardware_domain(d);
-}
+bool arch_acquire_resource_check(const struct domain *d);
 
 /*
  * Updates vCPU's n2pm to match its np2m_base in VMCx12 and returns that np2m.
@@ -720,6 +713,10 @@ p2m_pod_offline_or_broken_hit(struct page_info *p);
 void
 p2m_pod_offline_or_broken_replace(struct page_info *p);
 
+/* Perform cleanup of p2m mappings ahead of teardown. */
+int
+relinquish_p2m_mapping(struct domain *d);
+
 #else
 
 static inline bool
@@ -748,6 +745,11 @@ static inline void p2m_pod_offline_or_broken_replace(struct page_info *p)
     ASSERT_UNREACHABLE();
 }
 
+static inline int relinquish_p2m_mapping(struct domain *d)
+{
+    return 0;
+}
+
 #endif
 
 
@@ -1043,7 +1045,7 @@ static inline int p2m_entry_modify(struct p2m_domain *p2m, p2m_type_t nt,
         break;
 
     case p2m_map_foreign:
-        if ( !mfn_valid(nfn) )
+        if ( !mfn_valid(nfn) || p2m != p2m_get_hostp2m(p2m->domain) )
         {
             ASSERT_UNREACHABLE();
             return -EINVAL;
@@ -1068,7 +1070,7 @@ static inline int p2m_entry_modify(struct p2m_domain *p2m, p2m_type_t nt,
         break;
 
     case p2m_map_foreign:
-        if ( !mfn_valid(ofn) )
+        if ( !mfn_valid(ofn) || p2m != p2m_get_hostp2m(p2m->domain) )
         {
             ASSERT_UNREACHABLE();
             return -EINVAL;
diff --git a/xen/arch/x86/mm/p2m-basic.c b/xen/arch/x86/mm/p2m-basic.c
index 8599bd15c61a..25d27a0a9f56 100644
--- a/xen/arch/x86/mm/p2m-basic.c
+++ b/xen/arch/x86/mm/p2m-basic.c
@@ -13,6 +13,8 @@
 
 #include <xen/event.h>
 #include <xen/types.h>
+#include <asm/altp2m.h>
+#include <asm/hvm/nestedhvm.h>
 #include <asm/p2m.h>
 #include "mm-locks.h"
 #include "p2m.h"
@@ -207,6 +209,22 @@ void p2m_final_teardown(struct domain *d)
     p2m_teardown_hostp2m(d);
 }
 
+bool arch_acquire_resource_check(const struct domain *d)
+{
+    /*
+     * altp2m is not supported as we would otherwise also need to walk the
+     * altp2m tables and drop any foreign map entries in order to drop the page
+     * reference.
+     *
+     * The same applies to nestedhvm nested p2m tables, as the type from the L0
+     * p2m is replicated into the L1 p2m, and there's no filtering that
+     * prevents foreign mappings from being created in nestedp2m.
+     */
+    return is_pv_domain(d) ||
+           (d->arch.hvm.params[HVM_PARAM_ALTP2M] == XEN_ALTP2M_disabled &&
+            !nestedhvm_enabled(d));
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index ce742c12e0de..23c6c42af275 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -2333,10 +2333,6 @@ static int p2m_add_foreign(struct domain *tdom, unsigned long fgfn,
     int rc;
     struct domain *fdom;
 
-    /*
-     * hvm fixme: until support is added to p2m teardown code to cleanup any
-     * foreign entries, limit this to hardware domain only.
-     */
     if ( !arch_acquire_resource_check(tdom) )
         return -EPERM;
 
@@ -2693,6 +2689,70 @@ int p2m_set_altp2m_view_visibility(struct domain *d, unsigned int altp2m_idx,
     return rc;
 }
 
+/*
+ * Remove foreign mappings from the p2m, as that drops the page reference taken
+ * when mapped.
+ */
+int relinquish_p2m_mapping(struct domain *d)
+{
+    struct p2m_domain *p2m = p2m_get_hostp2m(d);
+    unsigned long gfn, count = 0;
+    int rc = 0;
+
+    if ( !paging_mode_translate(d) )
+        return 0;
+
+    BUG_ON(!d->is_dying);
+
+    p2m_lock(p2m);
+
+    gfn = p2m->teardown_gfn;
+
+    /* Iterate over the whole p2m on debug builds to ensure correctness. */
+    while ( gfn <= p2m->max_mapped_pfn &&
+            (IS_ENABLED(CONFIG_DEBUG) || p2m->nr_foreign) )
+    {
+        unsigned int order;
+        p2m_type_t t;
+        p2m_access_t a;
+
+        _get_gfn_type_access(p2m, _gfn(gfn), &t, &a, 0, &order, 0);
+        ASSERT(IS_ALIGNED(gfn, 1UL << order));
+
+        if ( t == p2m_map_foreign )
+        {
+            ASSERT(p2m->nr_foreign);
+            ASSERT(order == 0);
+
+            rc = p2m_set_entry(p2m, _gfn(gfn), INVALID_MFN, order, p2m_invalid,
+                               p2m->default_access);
+            if ( rc )
+            {
+                printk(XENLOG_ERR
+                       "%pd: failed to unmap foreign page %" PRI_gfn " order %u error %d\n",
+                       d, gfn, order, rc);
+                ASSERT_UNREACHABLE();
+                break;
+            }
+        }
+
+        gfn += 1UL << order;
+
+        if ( !(++count & 0xff) && hypercall_preempt_check() )
+        {
+            rc = -ERESTART;
+            break;
+        }
+    }
+
+    ASSERT(gfn <= p2m->max_mapped_pfn || !p2m->nr_foreign);
+    p2m->teardown_gfn = gfn;
+
+    p2m_unlock(p2m);
+
+    return rc;
+}
+
 /*
  * Local variables:
  * mode: C
-- 
2.44.0