[PATCH 14/21] KVM: SEV: Document that checking for SEV+ guests when reclaiming memory is "safe"

[Sean Christopherson]

Document that the check for an SEV+ guest when reclaiming guest memory is
safe even though kvm->lock isn't held.  This will allow asserting that
kvm->lock is held in the SEV accessors, without triggering false positives
on the "safe" cases.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/svm/sev.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 9a907bc057d0..7bb9318f0703 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3296,8 +3296,14 @@ void sev_guest_memory_reclaimed(struct kvm *kvm)
 	 * With SNP+gmem, private/encrypted memory is unreachable via the
 	 * hva-based mmu notifiers, i.e. these events are explicitly scoped to
 	 * shared pages, where there's no need to flush caches.
+	 *
+	 * Checking for SEV+ outside of kvm->lock is safe as __sev_guest_init()
+	 * can only be done before vCPUs are created, caches can be incoherent
+	 * if and only if a vCPU was run, and either this task will see the VM
+	 * as being SEV+ or the vCPU won't be to access the memory (because of
+	 * the in-progress invalidation).
 	 */
-	if (!sev_guest(kvm) || sev_snp_guest(kvm))
+	if (!____sev_guest(kvm) || ____sev_snp_guest(kvm))
 		return;
 
 	sev_writeback_caches(kvm);
-- 
2.53.0.473.g4a7958ca14-goog