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

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Date:	Thu, 5 Jun 2014 09:49:08 +0100
From:	Matt Fleming <matt.fleming@...el.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.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 5 June 2014 08:18, Borislav Petkov <bp@...en8.de> wrote:.
>
> How are you going to detect when to save/restore state? Do it
> unconditionally would probably be a no-no. Even with all that optimized
> XSAVE* fun.

(I'm not talking about the crypto async code because I'm not familiar with it)

For the EFI pstore case we'd only be using this newly allocated
context space if we can't do the usual FPU xsave dance. e.g. we'd be
adding a new feature specifically for the !irq_fpu_usable() case. Only
then would we do an unconditional save. It would be useful to get some
numbers for this but I don't think it would be too bad, especially
given that it's in a fatal crash handler state anyway.

I don't think it's worth going to the trouble solely for the EFI
pstore code, but if it can also be used for the crypto code it might
be worth a look.
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