Re: [PATCH 2/2] x86/emul: Emulate %cr8 accesses

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2025 18:41, Andrew Cooper wrote:
> Petr reports:
> 
>   (XEN) MMIO emulation failed (1): d12v1 64bit @ 0010:fffff8057ba7dfbf -> 45 
> 0f 20 c2 ...
> 
> during introspection.
> 
> This is MOV %cr8, which is wired up for hvm_mov_{to,from}_cr(); the VMExit
> fastpaths, but not for the full emulation slowpaths.
> 
> Xen's handling of %cr8 turns out to be quite wrong.  At a minimum, we need
> storage for %cr8 separate to APIC_TPR, and to alter intercepts based on
> whether the vLAPIC is enabled or not.  But that's more work than there is time
> for in the short term, so make a stopgap fix.
> 
> Extend hvmemul_{read,write}_cr() with %cr8 cases.  Unlike hvm_mov_to_cr(),
> hardware hasn't filtered out invalid values (#GP checks are ahead of
> intercepts), so introduce X86_CR8_VALID_MASK.
> 
> Reported-by: Petr Beneš <w1benny@xxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -2288,6 +2288,10 @@ static int cf_check hvmemul_read_cr(
>          val = curr->arch.hvm.guest_cr[reg];
>          break;
>  
> +    case 8:
> +        val = (vlapic_get_reg(vcpu_vlapic(curr), APIC_TASKPRI) & 0xf0) >> 4;

No new #define then to use MASK_EXTR() here and ...

> @@ -2333,6 +2337,17 @@ static int cf_check hvmemul_write_cr(
>          rc = hvm_set_cr4(val, true);
>          break;
>  
> +    case 8:
> +        if ( val & ~X86_CR8_VALID_MASK )
> +        {
> +            rc = X86EMUL_EXCEPTION;
> +            break;
> +        }
> +
> +        vlapic_set_reg(vcpu_vlapic(curr), APIC_TASKPRI, val << 4);

... MASK_INSR() here?

Jan