linux-kernel - [PATCH] KVM: nSVM: clarify recalc

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Date:   Thu, 29 Dec 2022 15:59:46 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: [PATCH] KVM: nSVM: clarify recalc_intercepts() wrt CR8

The mysterious comment "We only want the cr8 intercept bits of L1"
dates back to basically the introduction of nested SVM, back when
the handling of "less typical" hypervisors was very haphazard.
With the development of kvm-unit-tests for interrupt handling,
the same code grew another vmcb_clr_intercept for the interrupt
window (VINTR) vmexit, this time with a comment that is at least
decent.

It turns out however that the same comment applies to the CR8 write
intercept, which is also a "recheck if an interrupt should be
injected" intercept.  The CR8 read intercept instead has not
been used by KVM for 14 years (commit 649d68643ebf, "KVM: SVM:
sync TPR value to V_TPR field in the VMCB"), so do not bother
clearing it and let one comment describe both CR8 write and VINTR
handling.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/nested.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index bc9cd7086fa9..add65dd59756 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -138,15 +138,13 @@ void recalc_intercepts(struct vcpu_svm *svm)
 		c->intercepts[i] = h->intercepts[i];

 	if (g->int_ctl & V_INTR_MASKING_MASK) {
-		/* We only want the cr8 intercept bits of L1 */
-		vmcb_clr_intercept(c, INTERCEPT_CR8_READ);
-		vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);
-
 		/*
-		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF does not
-		 * affect any interrupt we may want to inject; therefore,
-		 * interrupt window vmexits are irrelevant to L0.
+		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF and CR8
+		 * does not affect any interrupt we may want to inject;
+		 * therefore, writes to CR8 are irrelevant to L0, as are
+		 * interrupt window vmexits.
 		 */
+		vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);
 		vmcb_clr_intercept(c, INTERCEPT_VINTR);
 	}

-- 
2.31.1