Re: [PATCH 4/6] nestedsvm: Disable TscRateMSR

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.02.2024 02:20, George Dunlap wrote:
> For now, just disable the functionality entirely until we can
> implement it properly:
> 
> - Don't set TSCRATEMSR in the host CPUID policy

This goes too far: This way you would (in principle) also affect guests
with nesting disabled. According to the earlier parts of the description
there's also no issue with it in that case. What you want to make sure
it that in the HVM policy the bit isn't set.

While presently resolving to cpu_has_svm_feature(), I think
cpu_has_tsc_ratio really ought to resolve to the host policy field.
Of course then requiring the host policy to reflect reality rather than
having what is "always emulated". IOW ...

> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -407,8 +407,7 @@ static void __init calculate_host_policy(void)
>                                 (1u << SVM_FEATURE_PAUSEFILTER) |
>                                 (1u << SVM_FEATURE_DECODEASSISTS));
>          /* Enable features which are always emulated. */
> -        p->extd.raw[0xa].d |= ((1u << SVM_FEATURE_VMCBCLEAN) |
> -                               (1u << SVM_FEATURE_TSCRATEMSR));
> +        p->extd.raw[0xa].d |= (1u << SVM_FEATURE_VMCBCLEAN);

... this likely wants replacing altogether by not overriding what we
found in hardware, which would apparently mean moving the two bit
masks to the earlier "clamping" expression.

But then of course Andrew may know of reasons why all of this is done
in calculate_host_policy() in the first place, rather than in HVM
policy calculation.

Jan