linux-kernel - Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT

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Message-ID: <ad6c9663-2d9d-cfbd-f10d-5745731488fa@intel.com>
Date:   Thu, 21 May 2020 14:37:17 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Tao Xu <tao3.xu@...el.com>, Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL
 unconditionally

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@...hat.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?