Re: [PATCH v3 1/2] x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands

[Sairaj Kodilkar]

On 8/26/2026 6:49 PM, Uros Bizjak wrote:
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> 
> On Wed, Aug 26, 2026 at 2:30 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>>
>> On 8/26/26 00:00, Sairaj Kodilkar wrote:
>>> Extend the existing user CMPXCHG helpers to support 16-byte operands on
>>> x86-64, using LOCK_PREFIX "cmpxchg16b".  This mirrors the existing
>>> __try_cmpxchg64_user_asm() / cmpxchg8b path provided for 32-bit kernels,
>>> where KVM needs an atomic compare-exchange wider than the generic
>>> cmpxchg helper can provide.
>>
>> Please take a good look at the Sashiko review:
>>
>> https://sashiko.dev/#/patchset/20260826070004.8100-2-sarunkod%40amd.com
>>
>> It looks like the "A" constraint isn't one that you can cleanly mirror
>> from cmpxchg8b => cmpxchg16b.
> 
> Actually, "+A" will work for 64bit targets, as long as the variable is
> 128-bit. The comment in asm.h applies to 64-bit values, where on
> 32-bit targets they fit in eax *and* edx, while on 64-bit targets, the
> 64-bit values fit into rax *or* rdx.
> 
> This is documented in GCC documentation:
> 
>      ‘A’
>           The ‘a’ and ‘d’ registers.  This class is used for
>           instructions that return double word results in the ‘ax:dx’
>           register pair.  Single word values will be allocated either in
>           ‘ax’ or ‘dx’.
> 
> And can be confirmed with e.g.:
> 
> __int128 val;
> 
> void foo (void)
> {
>   __int128 _v = val;
>   asm volatile ("" : "+A" (_v));
>   val = _v;
> }
> 
> which will fail compilation with -ffixed-rax or -ffixed-rdx.
> 
> That said, the approach with union of two 64-bit halves can lead to
> slightly better code, because the compiler splits the value earlier in
> the compilation pipeline.
> 
>> Uros, any chance you can give these a good once-over? This seems to be
>> just the kind of thing you've been fixing up lately. It would be nice to
>> get them right the first time.
> 
> Based on the above explanation, these *can* be copied from 32-bit asm
> patterns. Even "q" constraint will include all integer registers on
> 64-bit targets.

Yep, this is what I was trying to say in my reply to sashiko. Basically
+A can be safely used for 64 bit values on 32 bit machines and 128 bit
values on 64 bit machines.

Also if you prefer split one, I can do that as well.

Sairaj

> 
> Uros.