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Re: [PATCH] xen/mcelog: simplify MCE_GETCLEAR_FLAGS using xchg()
- To: Uros Bizjak <ubizjak@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 22 Jan 2026 15:21:53 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
- Delivery-date: Thu, 22 Jan 2026 14:22:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.01.2026 15:17, Uros Bizjak wrote:
> The MCE_GETCLEAR_FLAGS ioctl retrieves xen_mcelog.flags while
> atomically clearing it. This was previously implemented using a
> cmpxchg() loop.
>
> Replace the cmpxchg() loop with a single xchg(), which provides the
> same atomic get-and-clear semantics, avoids retry spinning under
> contention, and simplifies the code.
>
> The code on x86_64 improves from:
>
> 186: 8b 15 00 00 00 00 mov 0x0(%rip),%edx
> 18c: 89 d0 mov %edx,%eax
> 18e: f0 0f b1 0d 00 00 00 lock cmpxchg %ecx,0x0(%rip)
> 195: 00
> 196: 39 c2 cmp %eax,%edx
> 198: 75 ec jne 186 <...>
>
> to just:
>
> 186: 87 05 00 00 00 00 xchg %eax,0x0(%rip)
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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