linux-kernel - Re: [DISCUSSION] x86: In-Kernel Use of Extended General-Purpose Registers

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Message-ID: <20251126163047.GA3666665@noisy.programming.kicks-ass.net>
Date: Wed, 26 Nov 2025 17:30:47 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Chang S. Bae" <chang.seok.bae@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
Subject: Re: [DISCUSSION] x86: In-Kernel Use of Extended General-Purpose
 Registers

On Mon, Nov 24, 2025 at 09:32:23PM +0000, Chang S. Bae wrote:

> This follows how vector registers are used today in places like crypto
> routines. AVX state usage is bracketed by kernel_fpu_begin() /
> kernel_fpu_end(). EGPRs could be similarly used in a small bounded
> region.
> 
> Under this model:
> 
>   * No changes are needed to the existing XSTATE management API.
> 
>   * Preemption and softirqs would be disabled while EGPRs are live,
>     subsequently limiting usage to small regions.
> 
>   * This lends itself mostly to hand-written assembly, which is less
>     scalable for broader adoption.

IIRC it isn't hard to make kernel_fpu_begin/end() preemptible. It came
up with the last xsave rework -- mostly in the context of -rt, but
nobody ever picked it up and did it.