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

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Date:   Wed, 11 May 2022 16:04:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Uros Bizjak <ubizjak@...il.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, 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.

32-bit KVM is mostly useful for testing the mess that is nested NPT; an L1
hypervsior can use 32-bit paging for NPT, so KVM needs to at least make sure it
doesn't blow up if such a hypervisor is encountered.  But in terms of the performance
of 32-bit KVM, I doubt there is a person in the world that cares.