Re: [PATCH v3 1/3] KVM: x86/pmu: Disable guest PEBS on hybird cpu due to heterogeneity

[Like Xu]

On 31/1/2023 1:40 am, Sean Christopherson wrote:
> On Mon, Jan 30, 2023, Like Xu wrote:
> > On 20/1/2023 8:47 am, Sean Christopherson wrote:
> > > On Wed, Nov 09, 2022, Like Xu wrote:
> > > > From: Like Xu <likexu@xxxxxxxxxxx>
> > > >
> > > >   From vPMU enabling perspective, KVM does not have proper support for
> > > > hybird x86 core. The reported perf_capabilities value (e.g. the format
> > > > of pebs record) depends on the type of cpu the kvm-intel module is init.
> > > > When a vcpu of one pebs format migrates to a vcpu of another pebs format,
> > > > the incorrect parsing of pebs records by guest can make profiling data
> > > > analysis extremely problematic.
> > > >
> > > > The safe way to fix this is to disable this part of the support until the
> > > > guest recognizes that it is running on the hybird cpu, which is appropriate
> > > > at the moment given that x86 hybrid architectures are not heavily touted
> > > > in the data center market.
> > > >
> > > > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > > > ---
> > > >    arch/x86/kvm/vmx/capabilities.h | 4 +++-
> > > >    1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> > > > index cd2ac9536c99..ea0498684048 100644
> > > > --- a/arch/x86/kvm/vmx/capabilities.h
> > > > +++ b/arch/x86/kvm/vmx/capabilities.h
> > > > @@ -392,7 +392,9 @@ static inline bool vmx_pt_mode_is_host_guest(void)
> > > >    static inline bool vmx_pebs_supported(void)
> > > >    {
> > > > -	return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
> > > > +	return boot_cpu_has(X86_FEATURE_PEBS) &&
> > > > +	       !boot_cpu_has(X86_FEATURE_HYBRID_CPU) &&
> > > > +	       kvm_pmu_cap.pebs_ept;
> > >
> > > I assume the patch I just posted[*] to disable the vPMU entirely is sufficient, or
> >
> > AFAI, some developers doing client-side virtualization on a hybrid cpu will
> > specifically want vPMU,
> > in which case it makes perfect sense for KVM to expose common pmu
> > capabilities (not PEBS at the current) of big and little cores, such as the
> > most basic performance counter.
> >
> > > do we need this as well in order to hide X86_FEATURE_DS and X86_FEATURE_DTES64?
> >
> > I think we still need this diff. Better to prioritize this minor feature a
> > little bit for hungry users.
>
> That wasn't my question.  My question was whether or not wholesale disabling vPMU
> is sufficient to prevent issues with PEBS.  Unless we need this patch on top of
> disabling the vPMU, my strong preference is to disable vPMU, or at the very least
> make it off-by-default and require a explicit override.

OK and if so, just set global module parameter "enable_pmu=false" for HYBRID_CPU.
With "disable vPMU" diff, this patch should be dropped since 
kvm_pmu_cap.pebs_ept = 0.

> I agree that there are users that want to enable vPMU for hybrid CPUs, but as
> stated in the link below, that needs to be a dedicated enabling effort.  I don't
> see any reason to exempt PEBS from that.  E.g. isn't PEBS usable if userspace pins
> vCPUs to pCPUs and enumerates an accurate topology to the guest?

So for HYBRID_CPU, {pebs, lbr, basic PMU} would be disabled globally by KVM
until a dedicated effort enables them one by one in the near future.

Follow up with a rewritten diff, 20230131085031.88939-1-likexu@xxxxxxxxxxx

> > > [*] https://lore.kernel.org/all/20230120004051.2043777-1-seanjc@xxxxxxxxxx