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

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

On 5/21/2020 2:37 PM, Xiaoyao Li wrote:
> On 5/21/2020 1:28 PM, Tao Xu wrote:
>>
>>
>> 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.
> 
> I guess you typed "unconditionally" by mistake that you meant to say 
> "conditionally" in fact?

I am sorry, I mean:
By default, we don't expose WAITPKG to guest. For QEMU, we can use 
"-overcommit cpu-pm=on" to use WAITPKG.