linux-kernel - [PATCH] KVM: x86: Fix emulation in writing cr8

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Date:   Wed,  9 Feb 2022 14:24:28 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...el.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org
Subject: [PATCH] KVM: x86: Fix emulation in writing cr8

In emulation of writing to cr8, one of the lowest four bits in TPR[3:0]
is kept.

According to Intel SDM 10.8.6.1(baremetal scenario):
"APIC.TPR[bits 7:4] = CR8[bits 3:0], APIC.TPR[bits 3:0] = 0";

and SDM 28.3(use TPR shadow):
"MOV to CR8. The instruction stores bits 3:0 of its source operand into
bits 7:4 of VTPR; the remainder of VTPR (bits 3:0 and bits 31:8) are
cleared.";

so in KVM emulated scenario, clear TPR[3:0] to make a consistent behavior
as in other scenarios.

This doesn't impact evaluation and delivery of pending virtual interrupts
because processor does not use the processor-priority sub-class to
determine which interrupts to delivery and which to inhibit.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...el.com>
---
 arch/x86/kvm/lapic.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d7e6fde82d25..306025db9959 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2242,10 +2242,7 @@ void kvm_set_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu, u64 data)

 void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8)
 {
-	struct kvm_lapic *apic = vcpu->arch.apic;
-
-	apic_set_tpr(apic, ((cr8 & 0x0f) << 4)
-		     | (kvm_lapic_get_reg(apic, APIC_TASKPRI) & 4));
+	apic_set_tpr(vcpu->arch.apic, (cr8 & 0x0f) << 4);
 }

 u64 kvm_lapic_get_cr8(struct kvm_vcpu *vcpu)
-- 
2.25.1