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

[Xiaoyao Li]

On 6/17/2019 11:50 PM, Radim KrÄmÃÅ 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:
> > > > +ÂÂÂ if (vmx->msr_ia32_umwait_control != host_umwait_control)
> > > > +ÂÂÂÂÂÂÂ add_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL,
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ vmx->msr_ia32_umwait_control,
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ host_umwait_control, false);
> > >
> > > The bit 1 is reserved, at least, we need to do below to ensure not
> > > modifying the reserved bit:
> > >
> > >   ÂÂÂÂguest_val = (vmx->msr_ia32_umwait_control & ~BIT_ULL(1)) |
> > >   ÂÂÂÂÂÂÂÂÂÂÂ (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.