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

[David Laight]

From: Andy Lutomirski
> Sent: 20 March 2018 14:57
...
> I'd rather see us finally finish the work that Rik started to rework
> this differently.  I'd like kernel_fpu_begin() to look like:
> 
> if (test_thread_flag(TIF_NEED_FPU_RESTORE)) {
>   return; // we're already okay.  maybe we need to check
> in_interrupt() or something, though?
> } else {
>   XSAVES/XSAVEOPT/XSAVE;
>   set_thread_flag(TIF_NEED_FPU_RESTORE):
> }
> 
> and kernel_fpu_end() does nothing at all.

I guess it might need to set (clear?) the CFLAGS bit for a process
that isn't using the fpu at all - which seems a sensible feature.
 
> We take the full performance hit for a *single* kernel_fpu_begin() on
> an otherwise short syscall or interrupt, but there's no additional
> cost for more of them or for long-enough-running things that we
> schedule in the middle.

It might be worth adding a parameter to kernel_fpu_begin() to indicate
which registers are needed, and a return value to say what has been
granted.
Then a driver could request AVX2 (for example) and use a fallback path
if the register set isn't available (for any reason).
A call from an ISR could always fail.

> As I remember, the main hangup was that this interacts a bit oddly
> with PKRU, but that's manageable.

WTF PKRU ??

	Dvaid