linux-kernel - [PATCH v14 17/22] KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP

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Message-ID: <20240421180122.1650812-18-michael.roth@amd.com>
Date: Sun, 21 Apr 2024 13:01:17 -0500
From: Michael Roth <michael.roth@....com>
To: <kvm@...r.kernel.org>
CC: <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
	<linux-crypto@...r.kernel.org>, <x86@...nel.org>,
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	<pankaj.gupta@....com>, <liam.merwick@...cle.com>
Subject: [PATCH v14 17/22] KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP

From: Ashish Kalra <ashish.kalra@....com>

With SNP/guest_memfd, private/encrypted memory should not be mappable,
and MMU notifications for HVA-mapped memory will only be relevant to
unencrypted guest memory. Therefore, the rationale behind issuing a
wbinvd_on_all_cpus() in sev_guest_memory_reclaimed() should not apply
for SNP guests and can be ignored.

Signed-off-by: Ashish Kalra <ashish.kalra@....com>
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
[mdr: Add some clarifications in commit]
Signed-off-by: Michael Roth <michael.roth@....com>
---
 arch/x86/kvm/svm/sev.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 243369e302f4..cf00a811aca5 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3042,7 +3042,14 @@ static void sev_flush_encrypted_page(struct kvm_vcpu *vcpu, void *va)
 
 void sev_guest_memory_reclaimed(struct kvm *kvm)
 {
-	if (!sev_guest(kvm))
+	/*
+	 * With SNP+gmem, private/encrypted memory should be
+	 * unreachable via the hva-based mmu notifiers. Additionally,
+	 * for shared->private translations, H/W coherency will ensure
+	 * first guest access to the page would clear out any existing
+	 * dirty copies of that cacheline.
+	 */
+	if (!sev_guest(kvm) || sev_snp_guest(kvm))
 		return;
 
 	wbinvd_on_all_cpus();
-- 
2.25.1