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

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Message-ID: <CAFULd4bZDO5-3T4q9fanHFrRTDj8v6fypiTc=dFPO9Rp61g9eQ@mail.gmail.com>
Date:   Wed, 11 May 2022 21:54:34 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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 Wed, May 11, 2022 at 6:04 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, May 11, 2022, Uros Bizjak wrote:
> > On Wed, May 11, 2022 at 9:54 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > Still, does 32bit actually support that stuff?
> >
> > Unfortunately, it does:
> >
> > kvm-intel-y        += vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
> >                vmx/evmcs.o vmx/nested.o vmx/posted_intr.o
> >
> > And when existing cmpxchg64 is substituted with cmpxchg, the
> > compilation dies for 32bits with:
>
> ...
>
> > > Anyway, your patch looks about right, but I find it *really* hard to
> > > care about 32bit code these days.
> >
> > Thanks, this is also my sentiment, but I hope the patch will enable
> > better code and perhaps ease similar situation I have had elsewhere.
>
> IMO, if we merge this it should be solely on the benefits to 64-bit code.  Yes,
> KVM still supports 32-bit kernels, but I'm fairly certain the only people that
> run 32-bit KVM are KVM developers.  32-bit KVM has been completely broken for
> multiple releases at least once, maybe twice, and no one ever complained.

Yes, the idea was to improve cmpxchg64 with the implementation of
try_cmpxchg64 for 64bit targets. However, the issue with 32bit targets
stood in the way, so the effort with 32-bit implementation was mainly
to unblock progression for 64-bit targets.

Uros.