linux-kernel - [PATCH v10 42/50] KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP

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Message-ID: <20231016132819.1002933-43-michael.roth@amd.com>
Date:   Mon, 16 Oct 2023 08:28:11 -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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Subject: [PATCH v10 42/50] 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>
[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 6c6d5a320d72..f027def3a79e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2852,7 +2852,14 @@ 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