From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <edumazet@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH] entry: always inline local_irq_{enable,disable}_exit_to_user()
Date: Fri, 5 Dec 2025 16:10:51 +0100 [thread overview]
Message-ID: <20251205151051.GB2528459@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <CANn89i+MiCZdTJJRaKHTkmu7DTuXVvqgCLi+YBpx3UqybOZ5zA@mail.gmail.com>
On Fri, Dec 05, 2025 at 05:03:33AM -0800, Eric Dumazet wrote:
> > Earlier Zen with RET mitigation? ;-)
>
> This was AMD Rome "AMD EPYC 7B12 64-Core Processor",
> bu also AMDTurin "AMD EPYC 9B45 128-Core Processor" to a certain extent.
>
> When you say RET mitigation, this is the five int3 after retq, right ?
Nope, that one is SLS, AMD has BTB type confusion on return prediction
(the AMD RetBleed) and patches all the RET sites with jumps to
retbleed_return_thunk(), or one of the srso*return_thunk() thingies. All
are somewhat expensive.
So while normally CALL+RET is well optimized and hardly noticeable, the
moment your uarch needs one of these return thunks, you're going to
notice them.
next prev parent reply other threads:[~2025-12-05 15:10 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-04 15:31 Eric Dumazet
2025-12-05 10:51 ` Peter Zijlstra
2025-12-05 10:54 ` Eric Dumazet
2025-12-05 12:45 ` Peter Zijlstra
2025-12-05 13:03 ` Eric Dumazet
2025-12-05 15:10 ` Peter Zijlstra [this message]
2025-12-18 9:45 ` [tip: core/urgent] entry: Always " tip-bot2 for Eric Dumazet
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