On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 2:54 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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.
