On Thu, Jun 05, 2014 at 09:31:12AM -0700, H. Peter Anvin wrote:
> The bottom line is that we can't call EFI from a context where we can't
> use the FPU. Or specifically, we can't then resume execution.
Can't we allocate a save-state area, stash the state there and let EFI
scribble over it? When EFI returns, we scribble over it back assuming it
has done the saving/restoring on its own.
> If all we're doing is stashing away some data before dying, well,
> then, by all means - but we need to make sure that is what actually
> happens.
Yeah, who knows, we might return. I'm thinking of a #MC here which is
serious enough to real exception, we do some handling and issue the
error info into pstore and continue execution. Purely hypothetical
though.
> As far adding additional xstate save areas, the current size of the
> xstate is about ~2.5K for AVX-512 enabled processors, and we need one
> per thread. If we make that two copies, then
> kernel_fpu_begin()..._end() would no longer have to disable preemption,
> but it wouldn't resolve the conflict about using the FPU from IRQ
> context when inside kernel_fpu_begin().._end().
>
> To support the FPU in IRQ context we end up having to create a percpu
> FPU state stack, and it becomes then a matter of how deep that stack
> would have to be.
... if it all makes sense at all, of course.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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