linux-kernel - Re: [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks

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Date:   Fri, 27 Aug 2021 09:44:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.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>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks

On Fri, Aug 27, 2021 at 02:52:25PM +0800, Like Xu wrote:
> + STATIC BRANCH/CALL friends.
> 
> On 27/8/2021 8:57 am, Sean Christopherson wrote:
> > This started out as a small series[1] to fix a KVM bug related to Intel PT
> > interrupt handling and snowballed horribly.
> > 
> > The main problem being addressed is that the perf_guest_cbs are shared by
> > all CPUs, can be nullified by KVM during module unload, and are not
> > protected against concurrent access from NMI context.
> 
> Shouldn't this be a generic issue of the static_call() usage ?
> 
> At the beginning, we set up the static entry assuming perf_guest_cbs != NULL:
> 
> 	if (perf_guest_cbs && perf_guest_cbs->handle_intel_pt_intr) {
> 		static_call_update(x86_guest_handle_intel_pt_intr,
> 				   perf_guest_cbs->handle_intel_pt_intr);
> 	}
> 
> and then we unset the perf_guest_cbs and do the static function call like this:
> 
> DECLARE_STATIC_CALL(x86_guest_handle_intel_pt_intr,
> *(perf_guest_cbs->handle_intel_pt_intr));
> static int handle_pmi_common(struct pt_regs *regs, u64 status)
> {
> 		...
> 		if (!static_call(x86_guest_handle_intel_pt_intr)())
> 			intel_pt_interrupt();
> 		...
> }

You just have to make sure all static_call() invocations that started
before unreg are finished before continuing with the unload.
synchronize_rcu() can help with that.

This is module unload 101. Nothing specific to static_call().