[PATCH 6/9] x86/p2m: limit cache flush in memory_type_changed()

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

Only do the cache flush when there's a p2m type change to propagate,
otherwise there's no change in the p2m effective caching attributes.

If the p2m memory_type_changed hook is not set p2m_memory_type_changed() is
a no-op, no recalculation of caching attributes is needed, nor flushing of
the previous cache contents.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/hvm/mtrr.c        |  3 +--
 xen/arch/x86/include/asm/p2m.h |  2 +-
 xen/arch/x86/mm/p2m.c          | 13 ++++++++++++-
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index b88e81eb44b1..e7acfb6e8dc4 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -767,9 +767,8 @@ 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)) &&
-         d->vcpu && d->vcpu[0] )
+         d->vcpu && d->vcpu[0] && p2m_memory_type_changed(d) )
     {
-        p2m_memory_type_changed(d);
         flush_all(FLUSH_CACHE);
     }
 }
diff --git a/xen/arch/x86/include/asm/p2m.h b/xen/arch/x86/include/asm/p2m.h
index b9ce7d8705ba..4358cc15a2a1 100644
--- a/xen/arch/x86/include/asm/p2m.h
+++ b/xen/arch/x86/include/asm/p2m.h
@@ -700,7 +700,7 @@ void p2m_pod_dump_data(struct domain *d);
 #ifdef CONFIG_HVM
 
 /* Report a change affecting memory types. */
-void p2m_memory_type_changed(struct domain *d);
+bool p2m_memory_type_changed(struct domain *d);
 
 /* Called by p2m code when demand-populating a PoD page */
 bool
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 3a39b5d1246b..b9a7c2dc5302 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -126,12 +126,21 @@ static void _memory_type_changed(struct p2m_domain *p2m)
 {
     if ( p2m->memory_type_changed )
         p2m->memory_type_changed(p2m);
+    else
+        ASSERT_UNREACHABLE();
 }
 
-void p2m_memory_type_changed(struct domain *d)
+bool p2m_memory_type_changed(struct domain *d)
 {
     struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
 
+    /*
+     * The p2m memory_type_changed hook will be the same for the host p2m or
+     * the altp2ms, do the check early and return if not set.
+     */
+    if ( !hostp2m->memory_type_changed )
+        return false;
+
     p2m_lock(hostp2m);
 
     _memory_type_changed(hostp2m);
@@ -154,6 +163,8 @@ void p2m_memory_type_changed(struct domain *d)
     }
 
     p2m_unlock(hostp2m);
+
+    return true;
 }
 
 int p2m_set_ioreq_server(struct domain *d,
-- 
2.48.1