Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: 2025-10-28 16:59:17
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On Tue, 28 Oct 2025 at 09:42, Kuniyuki Iwashima [off-list ref] wrote:
This is the Zen 2 platform, so probably the stac/clac cost will be more expensive than you expect on Zen 5.
Yeah, clac/stac are horrenously expensive on Zen 2. I think they are
microcoded - and not the fast kind - so effectively serializing.
They got enormously faster and pipelined in later Zen microarchitectures.
Sadly, Agner Fog doesn't do timings on those instructions, and I
haven't found any other place that has some overview of the
performance on different microarchitectures.
Linus