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

[Xiaoyao Li <xiaoyao.li@intel.com>]

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

> 
> in vmx_set_cpu_caps.
> 
> Thanks,
> 
> Paolo
>