Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally

[Tao Xu]

On 5/21/2020 12:33 PM, Xiaoyao Li wrote:
> On 5/21/2020 5:05 AM, 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>
> > > ---
> > > Â arch/x86/kvm/x86.c | 4 ++++
> > > Â 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)
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > +ÂÂÂÂÂÂÂÂÂÂÂ break;
> > > +ÂÂÂÂÂÂÂ case MSR_IA32_UMWAIT_CONTROL:
> > > +ÂÂÂÂÂÂÂÂÂÂÂ if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG))
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > ÂÂÂÂÂÂÂÂÂ default:
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > ÂÂÂÂÂÂÂÂÂ }
> >
> > 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
> >
> > ÂÂÂÂÂÂÂÂ if (vmx_waitpkg_supported())
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
>
> + Tao
>
> I remember there is certainly some reason why we don't expose WAITPKG to 
> guest by default.
>
> Tao, please help clarify it.
>
> Thanks,
> -Xiaoyao

Because in VM, umwait and tpause can put a (psysical) CPU into a power 
saving state. So from host view, this cpu will be 100% usage by VM. 
Although umwait and tpause just cause short wait(maybe 100 
microseconds), we still want to unconditionally expose WAITPKG in VM.