linux-kernel - Re: [RFC 8/9] RISC-V: KVM: Implement perf support

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Message-ID: <Y5C/4s7OannaS8+H@google.com>
Date:   Wed, 7 Dec 2022 16:31:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     Atish Patra <atishp@...osinc.com>, linux-kernel@...r.kernel.org,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>, Guo Ren <guoren@...nel.org>,
        kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC 8/9] RISC-V: KVM: Implement perf support

On Wed, Dec 07, 2022, Atish Patra wrote:
> On Fri, Dec 2, 2022 at 9:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Mon, Jul 18, 2022, Atish Patra wrote:
> > > RISC-V SBI PMU & Sscofpmf ISA extension allows supporting perf in
> > > the virtualization enviornment as well. KVM implementation
> > > relies on SBI PMU extension for most of the part while traps
> > > & emulates the CSRs read for counter access.
> >
> > For the benefit of non-RISCV people, the changelog (and documentation?) should
> > explain why RISC-V doesn't need to tap into kvm_register_perf_callbacks().
> 
> As per my understanding, kvm_register_perf_callbacks is only useful
> during event sampling for guests. Please let me know if I missed
> something.
> This series doesn't support sampling and guest counter overflow interrupt yet.
> That's why kvm_register_perf_callbacks support is missing.

Ah, I missed that connection in the cover letter.

In the future, if a patch adds partial support for a thing/feature, it's very
helpful to call that out in the lack shortlog and changelog, even for RFCs.  E.g.
adding a single word in the shortlog and sentence or two in the changelog doesn't
take much time on your end, and helps avoid cases like this where drive-by reviewers
like me from cause a fuss about non-issues.

 RISC-V: KVM: Implement partial perf support

 ...

 Counter overflow and interrupts are not supported as the relevant
 architectural specifications are still under discussion.

Thanks!