linux-kernel - Re: [RFC PATCH v2] x86/fpu: make kernel-mode FPU reliably usable in softirqs

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Message-ID: <20250306120947.GF16878@noisy.programming.kicks-ass.net>
Date: Thu, 6 Mar 2025 13:09:47 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Eric Biggers <ebiggers@...nel.org>, x86@...nel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Ben Greear <greearb@...delatech.com>,
	Xiao Liang <shaw.leon@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC PATCH v2] x86/fpu: make kernel-mode FPU reliably usable in
 softirqs

On Wed, Mar 05, 2025 at 10:07:45AM +0100, Ingo Molnar wrote:
> > From: Eric Biggers <ebiggers@...gle.com>

> > Alternatives considered:
> > 
> > - Make kernel-mode FPU sections fully preemptible.  This would require
> >   growing task_struct by another struct fpstate which is more than 2K.
> 
> So that's something that will probably happen once the kernel is built 
> using APX anyway?

ISTR looking into this at some point for RT. I also have vague memories
of other architectures doing something similar, but its all a long time
ago.