linux-kernel - [PATCH] KVM: SVM: Don't intercept IRET when injecting NMI and vNMI is enabled

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Message-ID: <20231009212919.221810-1-seanjc@google.com>
Date:   Mon,  9 Oct 2023 14:29:19 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Santosh Shukla <santosh.shukla@....com>
Subject: [PATCH] KVM: SVM: Don't intercept IRET when injecting NMI and vNMI is enabled

When vNMI is enabled, rely entirely on hardware to correctly handle NMI
blocking, i.e. don't intercept IRET to detect when NMIs are no longer
blocked.  KVM already correctly ignores svm->nmi_masked when vNMI is
enabled, so the effect of the bug is essentially an unnecessary VM-Exit.

Note, per the APM, hardware sets the BLOCKING flag when software directly
directly injects an NMI:

  If Event Injection is used to inject an NMI when NMI Virtualization is
  enabled, VMRUN sets V_NMI_MASK in the guest state.

Fixes: fa4c027a7956 ("KVM: x86: Add support for SVM's Virtual NMI")
Link: https://lore.kernel.org/all/ZOdnuDZUd4mevCqe@google.como
Cc: Santosh Shukla <santosh.shukla@....com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---

Santosh, can you verify that I didn't break vNMI?  I don't have access to the
right hardware.  Thanks!

 arch/x86/kvm/svm/svm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b7472ad183b9..4f22d12b5d60 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3569,8 +3569,15 @@ static void svm_inject_nmi(struct kvm_vcpu *vcpu)
 	if (svm->nmi_l1_to_l2)
 		return;
 
-	svm->nmi_masked = true;
-	svm_set_iret_intercept(svm);
+	/*
+	 * No need to manually track NMI masking when vNMI is enabled, hardware
+	 * automatically sets V_NMI_BLOCKING_MASK as appropriate, including the
+	 * case where software directly injects an NMI.
+	 */
+	if (!is_vnmi_enabled(svm)) {
+		svm->nmi_masked = true;
+		svm_set_iret_intercept(svm);
+	}
 	++vcpu->stat.nmi_injections;
 }
 

base-commit: 86701e115030e020a052216baa942e8547e0b487
-- 
2.42.0.609.gbb76f46606-goog