Re: [PATCH RESEND v3 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL

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

On 6/17/2019 11:50 PM, Radim Krčmář wrote:
> 2019-06-17 14:31+0800, Xiaoyao Li:
>> On 6/17/2019 11:32 AM, Xiaoyao Li wrote:
>>> On 6/16/2019 5:55 PM, Tao Xu wrote:
>>>> +    if (vmx->msr_ia32_umwait_control != host_umwait_control)
>>>> +        add_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL,
>>>> +                      vmx->msr_ia32_umwait_control,
>>>> +                      host_umwait_control, false);
>>>
>>> The bit 1 is reserved, at least, we need to do below to ensure not
>>> modifying the reserved bit:
>>>
>>>       guest_val = (vmx->msr_ia32_umwait_control & ~BIT_ULL(1)) |
>>>               (host_val & BIT_ULL(1))
>>>
>>
>> I find a better solution to ensure reserved bit 1 not being modified in
>> vmx_set_msr() as below:
>>
>> 	if((data ^ umwait_control_cached) & BIT_ULL(1))
>> 		return 1;
> 
> We could just be checking
> 
> 	if (data & BIT_ULL(1))
> 
> because the guest cannot change its visible reserved value and KVM
> currently initializes the value to 0.
> 
> The arch/x86/kernel/cpu/umwait.c series assumes that the reserved bit
> is 0 (hopefully deliberately) and I would do the same in KVM as it
> simplifies the logic.  (We don't have to even think about migrations
> between machines with a different reserved value and making it play
> nicely with possible future implementations of that bit.)
> 

Got it, thanks.

> Thanks.
>