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Date:   Thu, 29 Dec 2022 14:36:32 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Peter Zijlstra" <peterz@infradead.org>,
        "Linus Torvalds" <torvalds@linux-foundation.org>
Cc:     "Jonathan Corbet" <corbet@lwn.net>,
        "Will Deacon" <will@kernel.org>,
        "Boqun Feng" <boqun.feng@gmail.com>,
        "Mark Rutland" <mark.rutland@arm.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>, dennis@kernel.org,
        "Tejun Heo" <tj@kernel.org>, "Christoph Lameter" <cl@linux.com>,
        hca@linux.ibm.com, gor@linux.ibm.com,
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        borntraeger@linux.ibm.com, svens@linux.ibm.com,
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Subject: Re: [RFC][PATCH 07/12] percpu: Wire up cmpxchg128
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On Mon, Dec 19, 2022, at 16:35, Peter Zijlstra wrote:
> In order to replace cmpxchg_double() with the newly minted
> cmpxchg128() family of functions, wire it up in this_cpu_cmpxchg().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Does this work on x86 chips without X86_FEATURE_CX16?

As far as I can tell, the new percpu_cmpxchg128_op uses
the cmpxchg16b instruction unconditionally without checking
for the feature bit first, and is now used by this_cpu_cmpxchg()
unconditionally as well.

This is fine for the moment if the only user is mm/slub.c
and that retains the system_has_cmpxchg128() runtime check,
but I think a better interface would be to guarantee that
this_cpu_cmpxchg() always ends up either in a working
inline asm or the generic fallback but not an invalid
opcode.

For consistency, I would also suggest this_cpu_cmpxchg() to
take the same argument types as cmpxchg(): at most 'unsigned
long' sized, with additional this_cpu_cmpxchg64() and
this_cpu_cmpxchg128() macros that take fixed-size arguments.
I have an older patch set that additionally converts all
8-bit and 16-bit cmpxchg()/xchg() calls to cmpxchg_8()/
xchg_8()/cmpxchg_16()/xchg_16() and and leaves only the
fixed-32bit and variable typed 'unsigned long' sized
callers for the weakly typed variant.

       Arnd