Re: [PATCH v4 24/38] KVM: x86/pmu: Exclude PMU MSRs in vmx_get_passthrough_msr_slot()

[Sean Christopherson <seanjc@google.com>]

On Fri, May 16, 2025, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> > Reject PMU MSRs interception explicitly in
> > vmx_get_passthrough_msr_slot() since interception of PMU MSRs are
> > specially handled in intel_passthrough_pmu_msrs().
> > 
> > Signed-off-by: Mingwei Zhang <mizhang@google.com>
> > Co-developed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> > Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 38ecf3c116bd..7bb16bed08da 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -165,7 +165,7 @@ module_param(allow_smaller_maxphyaddr, bool, S_IRUGO);
> >  
> >  /*
> >   * List of MSRs that can be directly passed to the guest.
> > - * In addition to these x2apic, PT and LBR MSRs are handled specially.
> > + * In addition to these x2apic, PMU, PT and LBR MSRs are handled specially.

Except y'all forgot to actually do the "special" handling, vmx_msr_filter_changed()
needs to refresh the PMU MSR filters.  Only the x2APIC MSRs are excluded from
userspace filtering (see kvm_msr_allowed()), everything else can be intercepted
by userespace.  E.g. if an MSR filter is set _before_ PMU refresh, KVM's behavior
will diverge from a filter that is set after PMU refresh.

> >   */
> >  static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
> >  	MSR_IA32_SPEC_CTRL,
> > @@ -691,6 +691,16 @@ static int vmx_get_passthrough_msr_slot(u32 msr)
> >  	case MSR_LBR_CORE_FROM ... MSR_LBR_CORE_FROM + 8:
> >  	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 8:
> >  		/* LBR MSRs. These are handled in vmx_update_intercept_for_lbr_msrs() */
> > +	case MSR_IA32_PMC0 ...
> > +		MSR_IA32_PMC0 + KVM_MAX_NR_GP_COUNTERS - 1:
> > +	case MSR_IA32_PERFCTR0 ...
> > +		MSR_IA32_PERFCTR0 + KVM_MAX_NR_GP_COUNTERS - 1:
> > +	case MSR_CORE_PERF_FIXED_CTR0 ...
> > +		MSR_CORE_PERF_FIXED_CTR0 + KVM_MAX_NR_FIXED_COUNTERS - 1:
> > +	case MSR_CORE_PERF_GLOBAL_STATUS:
> > +	case MSR_CORE_PERF_GLOBAL_CTRL:
> > +	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
> > +		/* PMU MSRs. These are handled in intel_passthrough_pmu_msrs() */
> >  		return -ENOENT;
> >  	}
> 
> This belongs in the patch that configures interception.  A better split would be
> to have an Intel patch and an AMD patch, 

I take that back, splitting the Intel and AMD logic makes is just as messy,
because the control logic is very much shared between VMX and SVM.