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

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Message-ID: <20140605163711.GG16642@pd.tnic>
Date:	Thu, 5 Jun 2014 18:37:11 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Matt Fleming <matt.fleming@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state
 in irq ctxt

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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