Re: [PATCH v3 2/2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM

[Peter Zijlstra]

On Wed, Nov 11, 2026 at 05:20:53AM -0800, Usama Arif wrote:
> Since commit 68674f94ffc9 ("x86: don't use REP_GOOD or ERMS for small
> memory copies"), memcpy() uses REP MOVSB only on FSRM CPUs, so ERMS CPUs
> without FSRM, such as Skylake and Cooper Lake, copy every size with
> the scalar loop. Large copies matter there: in Meta's x86 fleet, about a
> third of the time in memcpy_orig() is btrfs zstd decompression copying
> pages into and out of zstd's buffers.
> 
> Use REP MOVSB from 2048 bytes on ERMS CPUs. On a 1.4 GHz Cooper Lake, an
> aligned, cache-hot 4 KiB memcpy() then takes 66.8% less time (123.3 ns
> instead of 371.6 ns). In a single-vCPU guest, reading a synthetic
> 512 MiB btrfs zstd file takes 14.8% less time (median of five kernel
> pairs). Smaller copies keep the scalar loop: using REP MOVSB from
> 32 bytes made 32/64/128/256-byte copies
> 111.8%/68.0%/63.2%/15.6% slower.
> 
> Prepare the return value and RCX count before the FSRM alternative. On
> FSRM CPUs, REP MOVSB replaces the jump to memcpy_orig(), followed by a
> RET in ordinary text. ITS then selects return handling using the RET's
> execution address.
> 
> For ERMS CPUs, shift the scratch copy of the length in RDX after the
> 32-byte check. RCX already holds the count, so large copies need no
> reload. Copies below 32 bytes skip the check, which is a single NOP
> without ERMS.

So no real objection to these patches from me.

However, it did remind me of something I have on the TODO list for a
rainy afternoon, namely merge memcpy and rep_movs_alternative.

It makes very little sense to me that we have two (divergent) copies of
what is basically the same thing.