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

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Message-ID: <YAHT+zLiIg/oUygZ@google.com>
Date:   Fri, 15 Jan 2021 09:42:19 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Xu, Like" <like.xu@...el.com>
Cc:     Like Xu <like.xu@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi@...stfloor.org>, wei.w.wang@...el.com,
        luwei.kang@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI
 and inject it to guest

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@intel.com/
> 
> +
> 
> 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.

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.