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

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Message-ID: <20220511075409.GX76023@worktop.programming.kicks-ass.net>
Date:   Wed, 11 May 2022 09:54:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uros Bizjak <ubizjak@...il.com>
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 07:07:25PM +0200, Uros Bizjak wrote:
> 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].

I can't read that code, my brain is hard wired to read pi as priority
inheritance/inversion.

Still, does 32bit actually support that stuff?

> 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

tdp_mmu is definitely 64bit only and as such shouldn't need to use
cmpxchg64.


Anyway, your patch looks about right, but I find it *really* hard to
care about 32bit code these days.