RE: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

[Thomas Gleixner]

On Mon, 19 Mar 2018, David Laight wrote:
> From: Rahul Lakkireddy
> In principle it ought to be possible to get access to one or two
> (eg) AVX registers by saving them to stack and telling the fpu
> save code where you've put them.

No. We have functions for this and we are not adding new ad hoc magic.

> OTOH, for x86, if the code always runs in process context (eg from a
> system call) then, since the ABI defines them all as caller-saved
> the AVX(2) registers, it is only necessary to ensure that the current
> FPU registers belong to the current process once.
> The registers can be set to zero by an 'invalidate' instruction on
> system call entry (hope this is done) and after use.

Why would a system call touch the FPU registers? The kernel normally does
not use FPU instructions and the code which explicitely does has to take
care of save/restore. It would be performance madness to fiddle with the
FPU stuff unconditionally if nothing uses it.

Thanks,

	tglx