Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

[Mi, Dapeng]

On 4/12/2024 7:31 AM, Sean Christopherson wrote:
> On Thu, Apr 11, 2024, Jim Mattson wrote:
> > On Thu, Apr 11, 2024 at 2:44 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > > +     /* Clear host global_ctrl and global_status MSR if non-zero. */
> > > > +     wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, 0);
> > > Why?  PERF_GLOBAL_CTRL will be auto-loaded at VM-Enter, why do it now?
> > >
> > > > +     rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, global_status);
> > > > +     if (global_status)
> > > > +             wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status);
> > > This seems especially silly, isn't the full MSR being written below?  Or am I
> > > misunderstanding how these things work?
> > LOL! You expect CPU design to follow basic logic?!?
> >
> > Writing a 1 to a bit in IA32_PERF_GLOBAL_STATUS_SET sets the
> > corresponding bit in IA32_PERF_GLOBAL_STATUS to 1.
> >
> > Writing a 0 to a bit in to IA32_PERF_GLOBAL_STATUS_SET is a nop.
> >
> > To clear a bit in IA32_PERF_GLOBAL_STATUS, you need to write a 1 to
> > the corresponding bit in IA32_PERF_GLOBAL_STATUS_RESET (aka
> > IA32_PERF_GLOBAL_OVF_CTRL).
> If only C had a way to annotate what the code is doing. :-)
>
> > > > +     wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status);
> IIUC, that means this should be:
>
> 	if (pmu->global_status)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status);
>
> or even better:
>
> 	toggle = pmu->global_status ^ global_status;
> 	if (global_status & toggle)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status & toggle);
> 	if (pmu->global_status & toggle)
> 		wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status & toggle);
Thanks, it looks better. Since PMU v4+, the MSR 
CORE_PERF_GLOBAL_OVF_CTRL is renamed to CORE_PERF_GLOBAL_STATUS_RESET 
with supporting more bits. CORE_PERF_GLOBAL_STATUS_RESET looks more 
easily understand just from name than CORE_PERF_GLOBAL_OVF_CTRL, I would 
prefer use this name in next version.