linux-kernel - Re: [PATCH 2/3] x86/fpu: Remove the thread::fpu pointer

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Message-ID: <20240608101040.GA7947@redhat.com>
Date: Sat, 8 Jun 2024 12:10:40 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...1.net>, Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 2/3] x86/fpu: Remove the thread::fpu pointer

On 06/08, Ingo Molnar wrote:
>
> On a third thought, while more readable, this doesn't work in practice with
> the current scheme, because x86_task_fpu() gets called on kthreads in
> fpu_clone(), which trips up the new debugging code.

Yes, yes, I even mentioned this in reply to 3/3.

> We could resolve it by special-casing PF_KTHREAD here too, but that weakens
> the whole readability argument. I'll leave it as-is for now.

Agreed.

Note that PF_KTHREAD | PF_USER_WORKER is already a special case in fpu_clone(),
see the "if (minimal)" check. We can probably do more changes later, for example
I don't even understand why do we need to initialize dst_fpu->fpstate->regs,
switch_fpu_return() is only called by arch_exit_to_user_mode_prepare().

Oleg.