xen/x86: iommu: Ignore IOMMU mapping requests when a domain is dying

The new x86 IOMMU page-tables allocator will release the pages when
relinquishing the domain resources. However, this is not sufficient
when the domain is dying because nothing prevents page-table to be
allocated.

As the domain is dying, it is not necessary to continue to modify the
IOMMU page-tables as they are going to be destroyed soon.

At the moment, page-table allocates will only happen when iommu_map().
So after this change there will be no more page-table allocation
happening because we don't use superpage mappings yet when not sharing
page tables.

In order to observe d->is_dying correctly, we need to rely on per-arch
locking, so the check to ignore IOMMU mapping is added on the per-driver
map_page() callback.

Fixes: 15bc9a1ef5 ("x86/iommu: add common page-table allocator")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

xen/drivers/passthrough/amd/iommu_map.c

@@ -285,6 +285,18 @@ int amd_iommu_map_page(struct domain *d, dfn_t dfn, mfn_t mfn,
 
     spin_lock(&hd->arch.mapping_lock);
 
+    /*
+     * IOMMU mapping request can be safely ignored when the domain is dying.
+     *
+     * hd->arch.mapping_lock guarantees that d->is_dying will be observed
+     * before any page tables are freed (see iommu_free_pgtables()).
+     */
+    if ( d->is_dying )
+    {
+        spin_unlock(&hd->arch.mapping_lock);
+        return 0;
+    }
+
     rc = amd_iommu_alloc_root(d);
     if ( rc )
     {

xen/drivers/passthrough/vtd/iommu.c

@@ -1762,6 +1762,18 @@ static int __must_check intel_iommu_map_page(struct domain *d, dfn_t dfn,
 
     spin_lock(&hd->arch.mapping_lock);
 
+    /*
+     * IOMMU mapping request can be safely ignored when the domain is dying.
+     *
+     * hd->arch.mapping_lock guarantees that d->is_dying will be observed
+     * before any page tables are freed (see iommu_free_pgtables())
+     */
+    if ( d->is_dying )
+    {
+        spin_unlock(&hd->arch.mapping_lock);
+        return 0;
+    }
+
     pg_maddr = addr_to_dma_page_maddr(d, dfn_to_daddr(dfn), 1);
     if ( !pg_maddr )
     {

xen/drivers/passthrough/x86/iommu.c

@@ -270,6 +270,9 @@ int iommu_free_pgtables(struct domain *d)
     if ( !is_iommu_enabled(d) )
         return 0;
 
+    /* After this barrier, no new IOMMU mappings can be inserted. */
+    spin_barrier(&hd->arch.mapping_lock);
+
     while ( (pg = page_list_remove_head(&hd->arch.pgtables.list)) )
     {
         free_domheap_page(pg);