Re: [PATCH] x86: introduce "hot" and "cold" page copying functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 23:33, Jason Andryuk wrote:
> On 2025-08-28 05:17, Jan Beulich wrote:
>> The present copy_page_sse2() is useful in case the destination page isn't
>> going to get touched again soon, or if we want to limit churn on the
>> caches. Just rename it, to fit the corresponding {clear,scrub}_page_*()
>> naming scheme.
>>
>> For cases where latency is the most important aspect, or when it is
>> expected that sufficiently large parts of a destination page will get
>> accessed again soon after the copying, introduce a "hot" alternative.
>> Again use alternatives patching to select between a "legacy" and an ERMS
>> variant.
>>
>> Don't switch any callers just yet - this will be the subject of subsequent
>> changes.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>

Thanks.

>> To avoid the NOP padding (also in clear_page_hot()) we could use a double
>> REP prefix in the replacement code (accounting for the REX one in the code
>> being replaced).
> 
> Did my tool chain do it automatically?
> 
> 0000000000000000 <.altinstr_replacement>:
>     0:        b9 00 10 00 00          mov    $0x1000,%ecx
>     5:        f3 f3 a4                repz rep movsb %ds:(%rsi),%es:(%rdi)

Interesting. That looks like a bug to me, when source code merely has

        rep movsb

Did you also check what copy_page_movsq (i.e. "rep movsq") expands to?
What gas version is this? With 2.45 I get

0000000000000000 <.altinstr_replacement>:
   0:   b9 00 10 00 00          mov    $0x1000,%ecx
   5:   f3 a4                   rep movsb (%rsi),(%rdi)

(the omission of segment indicators when there's no segment override is
indeed a change in 2.45).

Jan