linux-kernel - Re: [PATCH v2 00/13] perf: KVM: Fix, optimize, and clean up callbacks

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Date:   Fri, 17 Sep 2021 16:53:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH v2 00/13] perf: KVM: Fix, optimize, and clean up callbacks

On Fri, Sep 17, 2021, Peter Zijlstra wrote:
> On Thu, Sep 16, 2021 at 09:37:43PM +0000, Sean Christopherson wrote:
> So I don't mind exporting __static_call_return0, but exporting a raw
> static_call is much like exporting a function pointer :/

Ya, that part is quite gross.

> > The unregister path would also need its own synchronize_rcu().  In general, I
> > don't love duplicating the logic, but it's not the end of the world.
> > 
> > Either way works for me.  Paolo or Peter, do either of you have a preference?
> 
> Can we de-feature kvm as a module and only have this PT functionality
> when built-in? :-)

I agree that many of the for-KVM exports are ugly, especially several of the
perf exports, but I will fight tooth and nail to keep KVM-as-a-module.  It is
invaluable for development and testing, and in the not-too-distant future there
is KVM-maintenance related functionality that we'd like to implement that relies
on KVM being a module.

I would be more than happy to help explore approaches that reduce the for-KVM
exports, but I am strongly opposed to defeaturing KVM-as-a-module.  I have a few
nascent ideas for eliminating a handful of a random exports, but no clever ideas
for eliminating perf's for-KVM exports.