linux-kernel - Re: [PATCH v5 00/25] context_tracking,x86: Defer some IPIs until a user->kernel transition

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Message-ID: <20250502112216.GZ4198@noisy.programming.kicks-ass.net>
Date: Fri, 2 May 2025 13:22:16 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
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Subject: Re: [PATCH v5 00/25] context_tracking,x86: Defer some IPIs until a
 user->kernel transition

On Wed, Apr 30, 2025 at 11:07:35AM -0700, Dave Hansen wrote:

> Both AMD and Intel have hardware to do it. ARM CPUs do it too, I think.
> You can go buy the Intel hardware off the shelf today.

To be fair, the Intel RAR thing is pretty horrific :-( Definitely
sub-par compared to the AMD and ARM things.

Furthermore, the paper states it is a uarch feature for SPR with no
guarantee future uarchs will get it (and to be fair, I'd prefer it if
they didn't).

Furthermore, I suspect it will actually be slower than IPIs for anything
with more than 64 logical CPUs due to reduced parallelism.