linux-kernel - Re: [RFC PATCH 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL

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Date: Thu, 11 Apr 2024 15:54:39 -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, 
	Xiong Zhang <xiong.y.zhang@...el.com>
Subject: Re: [RFC PATCH 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL_CTRL at
 VM boundary

On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 2:54 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > > +static void save_perf_global_ctrl_in_passthrough_pmu(struct vcpu_vmx *vmx)
> > > +{
> > > +     struct kvm_pmu *pmu = vcpu_to_pmu(&vmx->vcpu);
> > > +     int i;
> > > +
> > > +     if (vm_exit_controls_get(vmx) & VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL) {
> > > +             pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
> > > +     } else {
> > > +             i = vmx_find_loadstore_msr_slot(&vmx->msr_autostore.guest,
> > > +                                             MSR_CORE_PERF_GLOBAL_CTRL);
> > > +             if (i < 0)
> > > +                     return;
> > > +             pmu->global_ctrl = vmx->msr_autostore.guest.val[i].value;
> >
> > As before, NAK to using the MSR load/store lists unless there's a *really* good
> > reason I'm missing.
> 
> The VM-exit control, "save IA32_PERF_GLOBAL_CTL," first appears in
> Sapphire Rapids. I think that's a compelling reason.

Well that's annoying.  When was PMU v4 introduced?  E.g. if it came in ICX, I'd
be sorely tempted to make VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL a hard requirement.

And has someone confirmed that the CPU saves into the MSR store list before
processing VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL?

Assuming we don't make VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL a hard requirement,
this code should be cleaned up and simplified.  It should be impossible to get
to this point with a passthrough PMU and no slot for saving guest GLOBAL_CTRL.

E.g. this could simply be:

	if (cpu_has_save_perf_global_ctrl())
		pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
	else
		pmu->global_ctrl = *pmu->__global_ctrl;

where vmx_set_perf_global_ctrl() sets __global_ctrl to:

	pmu->__global_ctrl = &vmx->msr_autostore.guest.val[i].value;

KVM could store 'i', i.e. the slot, but in the end it's 4 bytes per vCPU (assuming
64-bit kernel, and an int to store the slot).

Oh, by the by, vmx_set_perf_global_ctrl() is buggy, as it neglects to *remove*
PERF_GLOBAL_CTRL from the lists if userspace sets CPUID multiple times.