[tip: x86/cpu] KVM: SVM: Don't flush cache if hardware enforces cache coherency across encryption domains

[tip-bot2 for Krish Sadhukhan]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     e1ebb2b49048c4767cfa0d8466f9c701e549fa5e
Gitweb:        https://git.kernel.org/tip/e1ebb2b49048c4767cfa0d8466f9c701e549fa5e
Author:        Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
AuthorDate:    Thu, 17 Sep 2020 21:20:38 
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Sat, 19 Sep 2020 20:46:59 +02:00

KVM: SVM: Don't flush cache if hardware enforces cache coherency across encryption domains

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
---
 arch/x86/kvm/svm/sev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 402dc42..567792f 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -384,7 +384,8 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
 	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++) {