linux-kernel - Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put

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Message-ID: <5bc35f89-1b77-4c0c-b6ea-fe9b2cb74870@citrix.com>
Date: Wed, 29 Oct 2025 01:42:42 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: torvalds@...ux-foundation.org
Cc: alex@...ti.fr, aou@...s.berkeley.edu, axboe@...nel.dk, bp@...en8.de,
 brauner@...nel.org, catalin.marinas@....com, christophe.leroy@...roup.eu,
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Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and
 unsafe_put_user() in epoll_put_uevent().

> Yeah, clac/stac are horrenously expensive on Zen 2. I think they are
> microcoded - and not the fast kind - so effectively serializing.

AIUI, Zen5 is the first uarch capable of renaming the AC flag.

This make STAC/CLAC "lfence-like" in practice on older CPUs, because of
an implementation constraint of not being able to execute speculatively.

~Andrew