Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

[Like Xu]

On 2021/1/16 1:42, Sean Christopherson wrote:
> On Fri, Jan 15, 2021, Xu, Like wrote:
> > On 2021/1/15 2:55, Sean Christopherson wrote:
> > > On Mon, Jan 04, 2021, Like Xu wrote:
> > > > +	 * Note: KVM disables the co-existence of guest PEBS and host PEBS.
> > > By "KVM", do you mean KVM's loading of the MSRs provided by intel_guest_get_msrs()?
> > > Because the PMU should really be the entity that controls guest vs. host.  KVM
> > > should just be a dumb pipe that handles the mechanics of how values are context
> > > switch.
> >
> > The intel_guest_get_msrs() and atomic_switch_perf_msrs()
> > will work together to disable the co-existence of guest PEBS and host PEBS:
> >
> > https://lore.kernel.org/kvm/961e6135-ff6d-86d1-3b7b-a1846ad0e4c4@xxxxxxxxx/
> >
> > +
> >
> > static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> > ...
> >      if (nr_msrs > 2 && (msrs[1].guest & msrs[0].guest)) {
> >          msrs[2].guest = pmu->ds_area;
> >          if (nr_msrs > 3)
> >              msrs[3].guest = pmu->pebs_data_cfg;
> >      }
> >
> >     for (i = 0; i < nr_msrs; i++)
> > ...
>
> Yeah, that's exactly what I'm complaining about.  Splitting the logic for
> determining the guest values is unnecessarily confusing, and as evidenced by the
> PEBS_ENABLE bug, potentially fragile.  Perf should have full knowledge and
> control of what values are loaded for the guest.  And, the above indexing magic
> is nigh impossible to follow and _super_ fragile.

Thanks for pointing this out.

> If we change .guest_get_msrs() to take a struct kvm_pmu pointer, then it can
> generate the full set of guest values by grabbing ds_area and pebs_data_cfg.
> Alternatively, .guest_get_msrs() could take the desired guest MSR values
> directly (ds_area and pebs_data_cfg), but kvm_pmu is vendor agnostic, so I don't
> see any reason to not just pass the pointer.

Hi Peter,

What do you think of us passing a "struct kvm_pmu" pointer (defined in 
arch/x86/include/asm/kvm_host.h) to guest_get_msrs(int *nr) ?

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thx,likexu