linux-kernel - Re: [PATCH v2 2/2] x86/fpu: Update the debug flow for x86_task

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Message-ID: <a27fbd35-aab5-4c48-9820-76832f706257@intel.com>
Date: Mon, 21 Jul 2025 18:40:04 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>
CC: Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Sean
 Christopherson" <seanjc@...gle.com>, Peter Zijlstra <peterz@...radead.org>,
	Vignesh Balasubramanian <vigbalas@....com>, Rick Edgecombe
	<rick.p.edgecombe@...el.com>, Oleg Nesterov <oleg@...hat.com>, "Chang S .
 Bae" <chang.seok.bae@...el.com>, Brian Gerst <brgerst@...il.com>, "Eric
 Biggers" <ebiggers@...gle.com>, Kees Cook <kees@...nel.org>, Chao Gao
	<chao.gao@...el.com>, Fushuai Wang <wangfushuai@...du.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] x86/fpu: Update the debug flow for x86_task_fpu()

On 7/21/2025 3:34 PM, Dave Hansen wrote:
> On 7/21/25 14:53, Sohil Mehta wrote:
>> Also, update the warning to include PF_USER_WORKER, as these tasks are
>> treated the same as kernel threads in the FPU context.
> 
> How so?

There are quite a few checks that special case PF_KTHREAD along with
PF_USER_WORKER. For example, FPSTATE allocation and freeing as well as
some context switch paths. Did I misinterpret something?

I believe it originates here:
https://lore.kernel.org/all/560c844c-f128-555b-40c6-31baff27537f@kernel.dk/

switch_fpu():
	if (!test_tsk_thread_flag(old, TIF_NEED_FPU_LOAD) &&
	    cpu_feature_enabled(X86_FEATURE_FPU) &&
	    !(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
		struct fpu *old_fpu = x86_task_fpu(old);


arch_release_task_struct():
	if (fpu_state_size_dynamic() && !(tsk->flags & (PF_KTHREAD |
PF_USER_WORKER)))
		fpstate_free(x86_task_fpu(tsk));


fpregs_restore_userregs():
if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER)))
	return;

kernel_fpu_begin_mask():
	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
		set_thread_flag(TIF_NEED_FPU_LOAD);
		save_fpregs_to_fpstate(x86_task_fpu(current));
	}