[PATCH 6.12.y 1/3] KVM: SEV: Make it more obvious when KVM is writing back the current PSC index

[Sasha Levin <sashal@kernel.org>]

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit 17d62e0b157f262dd8c7586cb882ece72a5b10c2 ]

Increment the guest-visible "cur_entry" index outside of the for-loop
when processing Page State Change entries, and add a comment to make it
more obvious which code is operating on trusted data, and which code is
touching guest-accessible data.

No functional change intended.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20260529183549.1104619-12-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Stable-dep-of: 98ade8c48c28 ("KVM: SEV: Mark vCPU RUNNABLE after AP_CREATE, even if VMSA is unusable")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/svm/sev.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index a6971cd236bed..e5587675cf9ec 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3821,7 +3821,7 @@ static int snp_begin_psc(struct vcpu_svm *svm)
 	}
 
 	/* Find the start of the next range which needs processing. */
-	for (idx = idx_start; idx <= idx_end; idx++, hdr->cur_entry++) {
+	for (idx = idx_start; idx <= idx_end; idx++) {
 		entry_start = READ_ONCE(entries[idx]);
 
 		gfn = entry_start.gfn;
@@ -3848,6 +3848,14 @@ static int snp_begin_psc(struct vcpu_svm *svm)
 
 		if (npages)
 			break;
+
+		/*
+		 * Increment the guest-visible index to communicate the current
+		 * entry back to the guest, e.g. in case of failure.  No need
+		 * for READ_ONCE() as KVM doesn't consume the field, i.e. a
+		 * misbehaving guest can only break itself.
+		 */
+		hdr->cur_entry++;
 	}
 
 	if (idx > idx_end) {
-- 
2.53.0