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Date:   Fri, 15 Jan 2021 13:01:26 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Xu, Like" <like.xu@intel.com>
Cc:     Like Xu <like.xu@linux.intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, eranian@google.com,
        kvm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Andi Kleen <andi@firstfloor.org>, wei.w.wang@intel.com,
        luwei.kang@intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI
 and inject it to guest
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On Thu, Jan 14, 2021 at 11:39:00AM +0800, Xu, Like wrote:

> > Why do we need to? Can't we simply always forward the PMI if the guest
> > has bits set in MSR_IA32_PEBS_ENABLE ? Surely we can access the guest
> > MSRs at a reasonable rate..
> > 
> > Sure, it'll send too many PMIs, but is that really a problem?
> 
> More vPMI means more guest irq handler calls and
> more PMI virtualization overhead.

Only if you have both guest and host PEBS. And in that case I really
can't be arsed about some overhead to the guest.

> In addition,
> the correctness of some workloads (RR?) depends on
> the correct number of PMIs and the PMI trigger times
> and virt may not want to break this assumption.

Are you sure? Spurious NMI/PMIs are known to happen anyway. We have far
too much code to deal with them.

> > > +	 * If PEBS interrupt threshold on host is not exceeded in a NMI, there
> > > +	 * must be a PEBS overflow PMI generated from the guest PEBS counters.
> > > +	 * There is no ambiguity since the reported event in the PMI is guest
> > > +	 * only. It gets handled correctly on a case by case base for each event.
> > > +	 *
> > > +	 * Note: KVM disables the co-existence of guest PEBS and host PEBS.
> > Where; I need a code reference here.
> 
> How about:
> 
> Note: KVM will disable the co-existence of guest PEBS and host PEBS.
> In the intel_guest_get_msrs(), when we have host PEBS ctrl bit(s) enabled,
> KVM will clear the guest PEBS ctrl enable bit(s) before vm-entry.
> The guest PEBS users should be notified of this runtime restriction.

Since you had me look at that function, can clean up that
CONFIG_RETPOLINE crud and replace it with static_call() ?