[PATCH v2 6/6] x86/hvm: reduce the need to flush caches in memory_type_changed()

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The current cache flushing done in memory_type_changed() is too wide, and
this doesn't scale on boxes with high number of CPUs.  Attempt to limit
cache flushes as a result of p2m type changes, and only do them if:

 * The CPU doesn't support (or has broken) self-snoop capability, otherwise
   there would be leftover aliases in the cache.  For guest initiated
   memory changes (like changes to MTRRs) the guest should already do a
   cache flush.
 * The IOMMU cannot force all DMA accesses to be snooping ones.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - New in this version.
---
Not sure whether this attempt to reduce cache flushing should get some
mention in the CHANGELOG.
---
 xen/arch/x86/hvm/mtrr.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index 8e1e15af8d73..9a6f6865a05e 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -782,14 +782,21 @@ HVM_REGISTER_SAVE_RESTORE(MTRR, hvm_save_mtrr_msr, NULL, 
hvm_load_mtrr_msr, 1,
 
 void memory_type_changed(struct domain *d)
 {
-    if ( (is_iommu_enabled(d) || cache_flush_permitted(d)) &&
+    if ( cache_flush_permitted(d) &&
          d->vcpu && d->vcpu[0] && p2m_memory_type_changed(d) &&
          /*
           * Do the p2m type-change, but skip the cache flush if the domain is
           * not yet running.  The check for creation_finished must strictly be
           * done after the call to p2m_memory_type_changed().
           */
-         d->creation_finished )
+         d->creation_finished &&
+         /*
+          * The cache flush should be done if either: CPU doesn't have
+          * self-snoop in which case there could be aliases left in the cache,
+          * or IOMMUs cannot force all DMA device accesses to be snooped.
+          */
+         (!boot_cpu_has(X86_FEATURE_XEN_SELFSNOOP) ||
+          (is_iommu_enabled(d) && !iommu_snoop)) )
     {
         flush_all(FLUSH_CACHE);
     }
-- 
2.48.1