linux-kernel - Re: [PATCH 05/15] perf: Track guest callbacks on a per-CPU basis

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Message-ID: <YSiRBQQE7md7ZrNC@hirez.programming.kicks-ass.net>
Date:   Fri, 27 Aug 2021 09:15:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        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 05/15] perf: Track guest callbacks on a per-CPU basis

On Thu, Aug 26, 2021 at 05:57:08PM -0700, Sean Christopherson wrote:
> Use a per-CPU pointer to track perf's guest callbacks so that KVM can set
> the callbacks more precisely and avoid a lurking NULL pointer dereference.

I'm completely failing to see how per-cpu helps anything here...

> On x86, KVM supports being built as a module and thus can be unloaded.
> And because the shared callbacks are referenced from IRQ/NMI context,
> unloading KVM can run concurrently with perf, and thus all of perf's
> checks for a NULL perf_guest_cbs are flawed as perf_guest_cbs could be
> nullified between the check and dereference.

No longer allowing KVM to be a module would be *AWESOME*. I detest how
much we have to export for KVM :/

Still, what stops KVM from doing a coherent unreg? Even the
static_call() proposed in the other patch, unreg can do
static_call_update() + synchronize_rcu() to ensure everybody sees the
updated pointer (would require a quick audit to see all users are with
preempt disabled, but I think your using per-cpu here already imposes
the same).