On Tue, Dec 04, 2018 at 01:15:10PM +0100, Sebastian Andrzej Siewior wrote:
> Do you refer to
> | * - by IRQ context code to potentially use the FPU
> | * if it's unused.
>
> ? It is possible to use the FPU in IRQ context.
I mean interrupted_user_mode() where we apparently can use the FPU when
handling an IRQ from user mode.
> The FPU could be used in user-context surrounded by kernel_fpu_begin().
Right, that.
> This only disables preemption so an IRQ could interrupt it. This IRQ
> could then use the FPU or raise a SoftIRQ which would use it.
> Therefore on x86 it is required to check with irq_fpu_usable() if the
Yes, and the check that thing does is:
return !in_interrupt() ||
interrupted_user_mode() || ...
so you're either *not* in interrupt, or you've gotten the IRQ while in
user mode.
> FPU can be used. If the FPU can not be used, you have to implement
> fallback code.
>
> With the "restore FPU on return to userland" series we need to modify
> the FPU in a few places. The softirq and preemption is disabled. I
> didn't find any in-IRQ users.
> Going forward I would like to remove the in-IRQ part and
> irq_fpu_usable() and disable softirq as part of kernel_fpu_begin().
Right, and we should document all those new conditions prominently so
that people are aware.
Thx.
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
