linux-kernel - Re: [PATCH 4/8] x86/fpu: Remove the thread::fpu pointer

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Message-ID: <20250410073948.GT9833@noisy.programming.kicks-ass.net>
Date: Thu, 10 Apr 2025 09:39:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave@...1.net>, Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Chang S . Bae" <chang.seok.bae@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 4/8] x86/fpu: Remove the thread::fpu pointer

On Wed, Apr 09, 2025 at 11:11:23PM +0200, Ingo Molnar wrote:

> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 5ea7e5d2c4de..b7f7c9c83409 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -514,12 +514,9 @@ struct thread_struct {
>  
>  	struct thread_shstk	shstk;
>  #endif
> -
> -	/* Floating point and extended processor state */
> -	struct fpu		*fpu;
>  };
>  
> -#define x86_task_fpu(task) ((task)->thread.fpu)
> +#define x86_task_fpu(task)	((struct fpu *)((void *)(task) + sizeof(*(task))))

Doesn't our FPU state need to be cacheline aligned? Looking at struct
fpu, it has a bunch of members that have __aligned__(64) on them, and as
such the whole of struct fpu will inherit this alignment requirement.

This means that both task and sizeof(*task) need to be cacheline aligned
for this to work.

Would it make sense to add something like:

static_assert(sizeof(struct task_struct) % 64 == 0);

And I didn't check, but isn't task a page pointer and as such always
page aligned?