linux-kernel - Re: [PATCH v2] KVM: x86: Fix emulation in writing cr8

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Date:   Thu, 24 Feb 2022 15:44:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Zhenzhong Duan <zhenzhong.duan@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com
Subject: Re: [PATCH v2] KVM: x86: Fix emulation in writing cr8

On 2/10/22 10:45, Zhenzhong Duan wrote:
> 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.";
> 
> and AMD's APM 16.6.4:
> "Task Priority Sub-class (TPS)-Bits 3 : 0. The TPS field indicates the
> current sub-priority to be used when arbitrating lowest-priority messages.
> This field is written with zero when TPR is written using the architectural
> CR8 register.";
> 
> 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.
> 
> Sub-class is used by hardware to arbitrate lowest priority interrupts,
> but KVM just does a round-robin style delivery.
> 
> Fixes: b93463aa59d6 ("KVM: Accelerated apic support")
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...el.com>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> ---
> v2: Add Sean's comments and "Fixes:" to patch description
> 
>   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)

Queued, thanks.

Paolo