linux-kernel - Re: [PATCH 3/9] x86/fpu: Make task

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Message-ID: <20240611124145.GA26798@redhat.com>
Date: Tue, 11 Jun 2024 14:42:13 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>, 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>, Brian Gerst <brgerst@...il.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uros Bizjak <ubizjak@...il.com>,
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH 3/9] x86/fpu: Make task_struct::thread constant size

I don't think this can explain the problem reported by Nathan, but.

On 06/08, Ingo Molnar wrote:
>
> +static struct fpu x86_init_fpu __read_mostly;
> +
>  static void __init fpu__init_system_early_generic(void)
>  {
> +	int this_cpu = smp_processor_id();
> +
> +	fpstate_reset(&x86_init_fpu);
> +	current->thread.fpu = &x86_init_fpu;

OK,

> +	per_cpu(fpu_fpregs_owner_ctx, this_cpu) = &x86_init_fpu;
> +	x86_init_fpu.last_cpu = this_cpu;

Why? I think it should do

	x86_init_fpu.last_cpu = -1;
	set_thread_flag(TIF_NEED_FPU_LOAD);

And the next patch should kill x86_init_fpu altogether, but keep
TIF_NEED_FPU_LOAD. It should be never cleared if PF_KTHREAD.

Oleg.