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

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Message-ID: <YSj9LQfbKxOhxqcP@hirez.programming.kicks-ass.net>
Date:   Fri, 27 Aug 2021 16:56:45 +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 Fri, Aug 27, 2021 at 02:49:50PM +0000, Sean Christopherson wrote:
> On Fri, Aug 27, 2021, Peter Zijlstra wrote:
> > 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...
> 
> It doesn't help until KVM is converted to set the per-cpu pointer in flows that
> are protected against preemption, and more specifically when KVM only writes to
> the pointer from the owning CPU.  

So the 'problem' I have with this is that sane (!KVM using) people, will
still have to suffer that load, whereas with the static_call() we patch
in an 'xor %rax,%rax' and only have immediate code flow.

> Ignoring static call for the moment, I don't see how the unreg side can be safe
> using a bare single global pointer.  There is no way for KVM to prevent an NMI
> from running in parallel on a different CPU.  If there's a more elegant solution,
> especially something that can be backported, e.g. an rcu-protected pointer, I'm
> all for it.  I went down the per-cpu path because it allowed for cleanups in KVM,
> but similar cleanups can be done without per-cpu perf callbacks.

If all the perf_guest_cbs dereferences are with preemption disabled
(IRQs disabled, IRQ context, NMI context included), then the sequence:

	WRITE_ONCE(perf_guest_cbs, NULL);
	synchronize_rcu();

Ensures that all prior observers of perf_guest_csb will have completed
and future observes must observe the NULL value.