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 11:02:06 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Matt Fleming <matt.fleming@...el.com>
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 Thu, Jun 05, 2014 at 09:49:08AM +0100, Matt Fleming wrote:
> 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.

Right, if we do this only for special, slowpath cases, then we're
probably fine with unconditional. It would be simpler too.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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