Re: [PATCH] KVM: x86: Fix emulation in writing cr8

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 09, 2022, 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.";
> 
> so in KVM emulated scenario, clear TPR[3:0] to make a consistent behavior
> as in other scenarios.

AMD's APM agrees:

  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.

> 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.

I believe hardware uses it to arbitrate lowest priority interrupts, but KVM just
does a round-robin style delivery.

> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>

Probably worth:

  Fixes: b93463aa59d6 ("KVM: Accelerated apic support")

Reviewed-by: Sean Christopherson <seanjc@google.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);

This appears to have been deliberate, but I've no idea what on earth it was
trying to do.  Preserving only bit 2 is super weird.

Author: Avi Kivity <avi@qumranet.com>
Date:   Thu Oct 25 16:52:32 2007 +0200

    KVM: Accelerated apic support

    This adds a mechanism for exposing the virtual apic tpr to the guest, and a
    protocol for letting the guest update the tpr without causing a vmexit if
    conditions allow (e.g. there is no interrupt pending with a higher priority
    than the new tpr).

    Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 50c3f3a8dd3d..e7513bb98af1 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -815,7 +815,8 @@ void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8)

        if (!apic)
                return;
-       apic_set_tpr(apic, ((cr8 & 0x0f) << 4));
+       apic_set_tpr(apic, ((cr8 & 0x0f) << 4)
+                    | (apic_get_reg(apic, APIC_TASKPRI) & 4));
 }