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Re: [PATCH] x86/alternatives: allow replacement code snippets to be re-used
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 30 Apr 2025 18:05:07 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 30 Apr 2025 16:05:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.04.2025 18:00, Andrew Cooper wrote:
> On 30/04/2025 2:13 pm, Jan Beulich wrote:
>> In a number of cases we use ALTERNATIVE_2 with both replacement insns /
>> insn sequences being identical. Avoid emitting the same code twice, and
>> instead alias the necessary helper labels to the existing ones.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> On a random build, the size of .altinstr_replacement drops from 0xe47 to
> 0xdf8, so not too bad.
>
> While the patch is fine, if we're adjusting the assembly ALTERNATIVE_2,
> we should make the same adjustment to the C version, even if there's
> nothing to benefit from it immediately.
Can do, but I expect that to end up more clumsy for, as you say, no real
gain.
> P.S. it would be even nicer if we would put these in mergeable sections,
> but I haven't figured out way to set the mergable unit size, which needs
> to be an absolute expression.
Well, have you missed
https://lists.xen.org/archives/html/xen-devel/2021-11/msg01009.html then?
Yet it might still make sense to use the approach here as well, as there
are limitations to what can really be merged (by the toolchain).
Jan
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