[PATCH 5.4 43/43] KVM: SVM: Dont flush cache if hardware enforces cache coherency across encryption domains

[Greg Kroah-Hartman]

From: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>

commit e1ebb2b49048c4767cfa0d8466f9c701e549fa5e upstream.

In some hardware implementations, coherency between the encrypted and
unencrypted mappings of the same physical page in a VM is enforced. In
such a system, it is not required for software to flush the VM's page
from all CPU caches in the system prior to changing the value of the
C-bit for the page.

So check that bit before flushing the cache.

Signed-off-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200917212038.5090-4-krish.sadhukhan@xxxxxxxxxx
[ The linux-5.4.y stable branch does not have the Linux 5.7 refactoring commit
  eaf78265a4ab ("KVM: SVM: Move SEV code to separate file") so the
  change was manually applied to sev_clflush_pages() in arch/x86/kvm/svm.c. ]
Signed-off-by: Liam Merwick <liam.merwick@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/svm.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1904,7 +1904,8 @@ static void sev_clflush_pages(struct pag
 	uint8_t *page_virtual;
 	unsigned long i;
 
-	if (npages == 0 || pages == NULL)
+	if (this_cpu_has(X86_FEATURE_SME_COHERENT) || npages == 0 ||
+	    pages == NULL)
 		return;
 
 	for (i = 0; i < npages; i++) {