[for-4.15][PATCH v4 1/2] xen/x86: iommu: Ignore IOMMU mapping requests when a domain is dying

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

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.

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.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

---

As discussed in v3, this is only covering 4.15. We can discuss
post-4.15 how to catch page-table allocations if another caller (e.g.
iommu_unmap() if we ever decide to support superpages) start to use the
page-table allocator.

Changes in v4:
    - Move the patch to the top of the queue
    - Reword the commit message

Changes in v3:
    - Patch added. This is a replacement of "xen/iommu: iommu_map: Don't
    crash the domain if it is dying"
---
 xen/drivers/passthrough/amd/iommu_map.c | 12 ++++++++++++
 xen/drivers/passthrough/vtd/iommu.c     | 12 ++++++++++++
 xen/drivers/passthrough/x86/iommu.c     |  6 ++++++
 3 files changed, 30 insertions(+)

diff --git a/xen/drivers/passthrough/amd/iommu_map.c 
b/xen/drivers/passthrough/amd/iommu_map.c
index d3a8b1aec766..560af54b765b 100644
--- a/xen/drivers/passthrough/amd/iommu_map.c
+++ b/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 )
     {
diff --git a/xen/drivers/passthrough/vtd/iommu.c 
b/xen/drivers/passthrough/vtd/iommu.c
index d136fe36883b..b549a71530d5 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/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 )
     {
diff --git a/xen/drivers/passthrough/x86/iommu.c 
b/xen/drivers/passthrough/x86/iommu.c
index cea1032b3d02..c6b03624fe28 100644
--- a/xen/drivers/passthrough/x86/iommu.c
+++ b/xen/drivers/passthrough/x86/iommu.c
@@ -267,6 +267,12 @@ int iommu_free_pgtables(struct domain *d)
     struct page_info *pg;
     unsigned int done = 0;
 
+    if ( !is_iommu_enabled(d) )
+        return 0;
+
+    /* After this barrier, no more IOMMU mapping can happen */
+    spin_barrier(&hd->arch.mapping_lock);
+
     while ( (pg = page_list_remove_head(&hd->arch.pgtables.list)) )
     {
         free_domheap_page(pg);
-- 
2.17.1