linux-kernel - Re: [PATCH] locking/atomic/x86: Introduce try

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Message-ID: <CAFULd4aNME5s2zGOO0A11kdjfHekH=ceSH7jUfAhmZaJWHv9cQ@mail.gmail.com>
Date:   Tue, 10 May 2022 19:07:25 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] locking/atomic/x86: Introduce try_cmpxchg64

On Tue, May 10, 2022 at 6:55 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, May 10, 2022 at 05:42:17PM +0200, Uros Bizjak wrote:
> > This patch adds try_cmpxchg64 to improve code around cmpxchg8b.  While
> > the resulting code improvements on x86_64 are minor (a compare and a move saved),
> > the improvements on x86_32 are quite noticeable. The code improves from:
>
> What user of cmpxchg64 is this?

This is cmpxchg64 in pi_try_set_control from
arch/x86/kvm/vmx/posted_intr.c, as shown in a RFC patch [1].

There are some more opportunities for try_cmpxchg64 in KVM, namely
fast_pf_fix_direct_spte in arch/x86/kvm/mmu/mmu.c and
tdp_mmu_set_spte_atomic in arch/x86/kvm/mmu/tdp_mmu.c

[1] https://www.spinics.net/lists/kvm/msg266042.html

Uros.