x86/fpu: Remove init_task FPU state dependencies, add debugging warning for PF_KTHREAD tasks

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init_task's FPU state initialization was a bit of a hack:

		__x86_init_fpu_begin = .;
		. = __x86_init_fpu_begin + 128*PAGE_SIZE;
		__x86_init_fpu_end = .;

But the init task isn't supposed to be using the FPU context
in any case, so remove the hack and add in some debug warnings.

As Linus noted in the discussion, the init task (and other
PF_KTHREAD tasks) *can* use the FPU via kernel_fpu_begin()/_end(),
but they don't need the context area because their FPU use is not
preemptible or reentrant, and they don't return to user-space.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20250409211127.3544993-8-mingo@kernel.org

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This commit is contained in:

Ingo Molnar 2025-04-09 23:11:26 +02:00

parent c360bdc593

commit 22aafe3bcb

5 changed files with 17 additions and 14 deletions

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6

arch/x86/include/asm/processor.h

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15

arch/x86/kernel/fpu/core.c

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3

arch/x86/kernel/fpu/init.c

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3

arch/x86/kernel/fpu/xstate.c

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4

arch/x86/kernel/vmlinux.lds.S

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