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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Thu, 6 Jun 2024 10:53:54 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave@sr71.net>, Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH 2/3] x86/fpu: Remove the thread::fpu pointer
Message-ID: <ZmF5IhYp5JiiMHgv@gmail.com>
References: <20240605083557.2051480-1-mingo@kernel.org>
 <20240605083557.2051480-3-mingo@kernel.org>
 <20240605133805.GB25006@redhat.com>
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* Oleg Nesterov <oleg@redhat.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:

        /*
         * We allocate the new FPU structure right after the end of the task struct.
         * task allocation size already took this into account.
         *
         * This is safe because task_struct size is a multiple of cacheline size.
         */
        struct fpu *dst_fpu = (void *)dst + sizeof(*dst);

        BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);

But yes, you are right, this is technically an open-coded x86_task_fpu().

Thanks,

	Ingo