On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 2:23 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > > From: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > >
> > > Intercept full-width GP counter MSRs in passthrough PMU if guest does not
> > > have the capability to write in full-width. In addition, opportunistically
> > > add a warning if non-full-width counter MSRs are also intercepted, in which
> > > case it is a clear mistake.
> > >
> > > Co-developed-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> > > Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
> > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/vmx/pmu_intel.c | 10 +++++++++-
> > > 1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > > index 7f6cabb2c378..49df154fbb5b 100644
> > > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > > @@ -429,6 +429,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > default:
> > > if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> > > (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> > > + if (is_passthrough_pmu_enabled(vcpu) &&
> > > + !(msr & MSR_PMC_FULL_WIDTH_BIT) &&
> > > + !msr_info->host_initiated) {
> > > + pr_warn_once("passthrough PMU never intercepts non-full-width PMU counters\n");
> > > + return 1;
> >
> > This is broken, KVM must be prepared to handle WRMSR (and RDMSR and RDPMC) that
> > come in through the emulator.
>
> Don't tell me that we are still supporting CPUs that don't have
> "unrestricted guest"! Sigh.
Heh, KVM still supports CPUs without VMX virtual NMIs :-)
Practically speaking, if we want to eliminate things like emulated WRMSR/RDMSR,
a Kconfig to build a reduced emulator would be the way to go. But while a reduced
emulator would be nice for host security, I don't think it would buy us much from
a code perspective, since KVM still needs to handle host userspace MSR accesses.
E.g. KVM could have conditional sanity checks for MSRs that are supposed to be
passed through, but unless a reduced emulator is a hard requirement for passthrough
PMUs, we'd still need the code to handle the emulated accesses. And even if a
reduced emulator were a hard requirement, I'd still push for a WARN-and-continue
approach, not a "inject a bogus #GP because KVM screwed up" approach.
