linux-kernel - Re: [PATCH 1/4] x86/percpu: Use explicit segment registers in lib/cmpxchg{8,16}b

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Date:   Thu, 12 Oct 2023 20:39:52 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] x86/percpu: Use explicit segment registers in lib/cmpxchg{8,16}b_emu.S

On Thu, Oct 12, 2023 at 7:54 PM Uros Bizjak <ubizjak@...il.com> wrote:
> > This will break on !SMP builds, where per-cpu variables are just
> > regular data and not accessed with a segment prefix.
>
> Ugh, indeed. Let me rethink this a bit.

Something like this:

#ifdef CONFIG_SMP
#define PER_CPU_ARG(arg)    %__percpu_seg:arg
#define PER_CPU_VAR(var)    %__percpu_seg:(var)##__percpu_rel
#else /* ! SMP */
#define PER_CPU_ARG(arg)    arg
#define PER_CPU_VAR(var)    (var)##__percpu_rel
#endif    /* SMP */

and using the above PER_CPU_ARG in /lib/cmpxchg{8,16}b_emu.S will
solve the issue.

I will prepare a v2.

Thanks,
Uros.