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

[Tao Xu <tao3.xu@intel.com>]

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@redhat.com>
>>> ---
>>>   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);
> 
> + 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.