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Date: Thu, 11 Apr 2024 16:27:34 -0700
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Subject: Re: [RFC PATCH 15/41] KVM: x86/pmu: Manage MSR interception for IA32_PERF_GLOBAL_CTRL
From: Sean Christopherson <seanjc@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: Xiong Zhang <xiong.y.zhang@linux.intel.com>, pbonzini@redhat.com, 
	peterz@infradead.org, mizhang@google.com, kan.liang@intel.com, 
	zhenyuw@linux.intel.com, dapeng1.mi@linux.intel.com, kvm@vger.kernel.org, 
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, 
	zhiyuan.lv@intel.com, eranian@google.com, irogers@google.com, 
	samantha.alt@intel.com, like.xu.linux@gmail.com, chao.gao@intel.com, 
	Xiong Zhang <xiong.y.zhang@intel.com>
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On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 2:21=E2=80=AFPM Sean Christopherson <seanjc@googl=
e.com> wrote:
> >
> > On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > > +     if (is_passthrough_pmu_enabled(&vmx->vcpu)) {
> > > +             /*
> > > +              * Setup auto restore guest PERF_GLOBAL_CTRL MSR at vm =
entry.
> > > +              */
> > > +             if (vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL)
> > > +                     vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL, 0);
> > > +             else {
> > > +                     i =3D vmx_find_loadstore_msr_slot(&vmx->msr_aut=
oload.guest,
> > > +                                                    MSR_CORE_PERF_GL=
OBAL_CTRL);
> > > +                     if (i < 0) {
> > > +                             i =3D vmx->msr_autoload.guest.nr++;
> > > +                             vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT,
> > > +                                          vmx->msr_autoload.guest.nr=
);
> > > +                     }
> > > +                     vmx->msr_autoload.guest.val[i].index =3D MSR_CO=
RE_PERF_GLOBAL_CTRL;
> > > +                     vmx->msr_autoload.guest.val[i].value =3D 0;
> >
> > Eww, no.   Just make cpu_has_load_perf_global_ctrl() and VM_EXIT_SAVE_I=
A32_PERF_GLOBAL_CTRL
> > hard requirements for enabling passthrough mode.  And then have clear_a=
tomic_switch_msr()
> > yell if KVM tries to disable loading MSR_CORE_PERF_GLOBAL_CTRL.
>=20
> Weren't you just complaining about the PMU version 4 constraint in
> another patch? And here, you are saying, "Don't support anything older
> than Sapphire Rapids."

Heh, I didn't realize VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL was SPR+ when I wr=
ote
this, I thought it existed alongside the "load" controls.

> Sapphire Rapids has PMU version 4, so if we require
> VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL, PMU version 4 is irrelevant.