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

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Message-ID: <ZmQAZ-REghlJmax-@gmail.com>
Date: Sat, 8 Jun 2024 08:55:35 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
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


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Oleg Nesterov <oleg@...hat.com> wrote:
> 
> > On 06/05, Ingo Molnar wrote:
> > >
> > > @@ -591,13 +591,11 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
> > >  	 * This is safe because task_struct size is a multiple of cacheline size.
> > >  	 */
> > >  	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > > -	struct fpu *src_fpu = current->thread.fpu;
> > > +	struct fpu *src_fpu = x86_task_fpu(current);
> > 
> > I think this patch can also change
> > 
> > 	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
> > 
> > above to use x86_task_fpu(dst).
> 
> Yeah, so I'd prefer to keep it open coded, because of the comment and the 
> debug check makes a lot more sense if the pointer calculation is visible:

On a second thought I changed it to your suggested variant:

        struct fpu *src_fpu = x86_task_fpu(current);
        struct fpu *dst_fpu = x86_task_fpu(dst);

because you are right, it's in fact easier to read this way.

Thanks,

	Ingo