Re: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Nov 30, 2016 at 01:38:28PM +0100, Florian Weimer wrote:

[...]

> We could add a system call to get the right stack size.  But as it depends
> on VL, I'm not sure what it looks like.  Particularly if you need determine
> the stack size before creating a thread that uses a specific VL setting.

I missed this point previously -- apologies for that.

What would you think of:

	set_vl(vl_for_new_thread);
	minsigstksz = get_minsigstksz();
	set_vl(my_vl);

This avoids get_minsigstksz() requiring parameters -- which is mainly a
concern because the parameters tomorrow might be different from the
parameters today.

If it is possible to create the new thread without any SVE-dependent code,
then we could

	set_vl(vl_for_new_thread);
	new_thread_stack = malloc(get_minsigstksz());
	new_thread = create_thread(..., new_thread_stack);
	set_vl(my_vl);

which has the nice property that the new thread directly inherits the
configuration that was used for get_minsigstksz().

However, it would be necessary to prevent GCC from moving any code
across these statements -- in particular, SVE code that access VL-
dependent data spilled on the stack is liable to go wrong if reordered
with the above.  So the sequence would need to go in an external
function (or a single asm...)

Failing that, we could maybe define some extensible struct to
get_minsigstksz().

Thoughts?

Cheers
---Dave