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
