On Wed, 2020-05-20 at 23:05 +0200, Paolo Bonzini wrote:
> On 20/05/20 18:07, Maxim Levitsky wrote:
> > This msr is only available when the host supports WAITPKG feature.
> >
> > This breaks a nested guest, if the L1 hypervisor is set to ignore
> > unknown msrs, because the only other safety check that the
> > kernel does is that it attempts to read the msr and
> > rejects it if it gets an exception.
> >
> > Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index fe3a24fd6b263..9c507b32b1b77 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void)
> > if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
> > min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> > continue;
> > + break;
> > + case MSR_IA32_UMWAIT_CONTROL:
> > + if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG))
> > + continue;
> > default:
> > break;
> > }
>
> The patch is correct, and matches what is done for the other entries of
> msrs_to_save_all. However, while looking at it I noticed that
> X86_FEATURE_WAITPKG is actually never added, and that is because it was
> also not added to the supported CPUID in commit e69e72faa3a0 ("KVM: x86:
> Add support for user wait instructions", 2019-09-24), which was before
> the kvm_cpu_cap mechanism was added.
>
> So while at it you should also fix that. The right way to do that is to
> add a
>
> if (vmx_waitpkg_supported())
> kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
>
> in vmx_set_cpu_caps.
>
> Thanks,
Thank you very much for finding this. I didn't expect this to be broken.
I will send a new version with this fix as well tomorrow.
Best regards,
Maxim Levitsky
>
> Paolo
>
