Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state in irq ctxt

[Borislav Petkov]

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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