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

Ingo Molnar
2015-07-17T08:39:56Z

* Peter Zijlstra <peterz@infradead.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