Re: [PATCH v3 01/57] KVM: x86: Use feature_bit() to clear CONSTANT_TSC when emulating CPUID

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> When clearing CONSTANT_TSC during CPUID emulation due to a Hyper-V quirk,
> use feature_bit() instead of SF() to ensure the bit is actually cleared.
> SF() evaluates to zero if the _host_ doesn't support the feature.  I.e.
> KVM could keep the bit set if userspace advertised CONSTANT_TSC despite
> it not being supported in hardware.

FWIW, I would strongly discourage such setups, all sorts of weird hangs
will likely be observed with Windows guests if TSC rate actually
changes.

>
> Note, translating from a scattered feature to a the hardware version is
> done by __feature_translate(), not SF().  The sole purpose of SF() is to
> check kernel support for the scattered feature, *before* translation.
>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 097bdc022d0f..776f24408fa3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1630,7 +1630,7 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>  				*ebx &= ~(F(RTM) | F(HLE));
>  		} else if (function == 0x80000007) {
>  			if (kvm_hv_invtsc_suppressed(vcpu))
> -				*edx &= ~SF(CONSTANT_TSC);
> +				*edx &= ~feature_bit(CONSTANT_TSC);
>  		}
>  	} else {
>  		*eax = *ebx = *ecx = *edx = 0;

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly