linux-kernel - Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'

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Date:	Fri, 17 Jul 2015 10:39:48 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dave Hansen <dave@...1.net>, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...en8.de, luto@...capital.net,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jul 16, 2015 at 12:14:37PM -0700, Dave Hansen wrote:
> > +++ b/arch/x86/kernel/fpu/init.c	2015-07-16 12:02:15.284280976 -0700
> > @@ -136,6 +136,45 @@ static void __init fpu__init_system_gene
> >  unsigned int xstate_size;
> >  EXPORT_SYMBOL_GPL(xstate_size);
> >  
> > +#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER)	\
> > +	BUILD_BUG_ON((sizeof(TYPE) -			\
> > +			offsetof(TYPE, MEMBER) -	\
> > +			sizeof(((TYPE *)0)->MEMBER)) > 	\
> > +			0)				\
> > +
> > +/*
> > + * We append the 'struct fpu' to the task_struct.
> > + */
> > +int __weak arch_task_struct_size(void)
> > +{
> > +	int task_size = sizeof(struct task_struct);
> > +
> > +	/*
> > +	 * Subtract off the static size of the register state.
> > +	 * It potentially has a bunch of padding.
> > +	 */
> > +	task_size -= sizeof(((struct task_struct *)0)->thread.fpu.state);
> > +
> > +	/*
> > +	 * Add back the dynamically-calculated register state
> > +	 * size.
> > +	 */
> > +	task_size += xstate_size;
> > +
> > +	/*
> > +	 * We dynamically size 'struct fpu', so we require that
> > +	 * it be at the end of 'thread_struct' and that
> > +	 * 'thread_struct' be at the end of 'task_struct'.  If
> > +	 * you hit a compile error here, check the structure to
> > +	 * see if something got added to the end.
> > +	 */
> > +	CHECK_MEMBER_AT_END_OF(struct fpu, state);
> > +	CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
> > +	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
> > +
> > +	return task_size;
> > +}
> 
> Since you want these invariants true at all times, maybe put the
> BUILD_BUG_ON() in generic code instead of x86 specific? That way people
> poking at other archs are less likely to accidentally break your stuff.

Yeah.

Thanks,

	Ingo
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