[PATCH] x86/svm: retry after unhandled NPT fault if gfn was marked for recalculation

[Igor Druzhinin <igor.druzhinin@citrix.com>]

If a recalculation NPT fault hasn't been handled explicitly in
hvm_hap_nested_page_fault() then it's potentially safe to retry -
US bit has been re-instated in PTE and any real fault would be correctly
re-raised next time.

This covers a specific case of migration with vGPU assigned on AMD:
global log-dirty is enabled and causes immediate recalculation NPT
fault in MMIO area upon access. This type of fault isn't described
explicitly in hvm_hap_nested_page_fault (this isn't called on
EPT misconfig exit on Intel) which results in domain crash.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
 xen/arch/x86/hvm/svm/svm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 46a1aac..f0d0bd3 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1726,6 +1726,10 @@ static void svm_do_nested_pgfault(struct vcpu *v,
         /* inject #VMEXIT(NPF) into guest. */
         nestedsvm_vmexit_defer(v, VMEXIT_NPF, pfec, gpa);
         return;
+    case 0:
+        /* If a recalculation page fault hasn't been handled - just retry. */
+        if ( pfec & PFEC_user_mode )
+            return;
     }
 
     /* Everything else is an error. */
-- 
2.7.4