linux-kernel - Re: [PATCH] KVM: vPMU: Don't program counter for interrupt-based event sampling w/o lapic_in

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Date:   Mon, 25 Oct 2021 18:44:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: vPMU: Don't program counter for interrupt-based
 event sampling w/o lapic_in_kernel

On 25/10/21 18:31, Sean Christopherson wrote:
>> vPMU depends on in-kernel lapic to deliver pmi interrupt, there is a
>> lot of overhead when creating/maintaining perf_event object,
>> locking/unlocking perf_event_ctx etc for vPMU. It silently fails to
>> deliver pmi interrupt if w/o in-kernel lapic currently. Let's not
>> program counter for interrupt-based event sampling w/o in-kernel
>> lapic support to avoid the whole bothering.
>
> This feels all kinds of wrong.  AFAIK, there's no way for KVM to enumerate to
> the guest that the vPMU isn't capable of generating interrupts.  I.e. any setup
> that exposes a vPMU to the guest without an in-kernel local APIC is either
> inherently broken or requires a paravirtualized guest.  I don't think KVM's bugs
> should be optimized.

Yeah, if it simplified the code it would be a different story, but here 
there's even not one but two new checks.

Paolo