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

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Date: Thu, 11 Apr 2024 16:31:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Xiong Zhang <xiong.y.zhang@...ux.intel.com>, pbonzini@...hat.com, 
	peterz@...radead.org, mizhang@...gle.com, kan.liang@...el.com, 
	zhenyuw@...ux.intel.com, dapeng1.mi@...ux.intel.com, kvm@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	zhiyuan.lv@...el.com, eranian@...gle.com, irogers@...gle.com, 
	samantha.alt@...el.com, like.xu.linux@...il.com, chao.gao@...el.com
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU

On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 2:44 PM Sean Christopherson <seanjc@...gle.com> 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);