linux-kernel - Re: [tip:x86/efi] x86/efi: Save and restore FPU context around efi_calls (x86

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Date:	Fri, 25 Apr 2014 12:09:36 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Borislav Petkov <bp@...e.de>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	ricardo.neri-calderon@...ux.intel.com, tglx@...utronix.de,
	matt.fleming@...el.com, linux-tip-commits@...r.kernel.org,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: Re: [tip:x86/efi] x86/efi: Save and restore FPU context around
 efi_calls (x86_64)

On Sun, 20 Apr, at 02:28:11AM, Borislav Petkov wrote:
> 
> I guess you can use the kernel_fpu_begin/end() variants here (i.e.,
> without the "__") which disable and enable preemption and thus drop the
> preempt_* calls:
> 
> 	efi_sync_low_kernel_mappings();
> 	kernel_fpu_begin();
> 	 __s = efi_call((void *)efi.systab->runtime->f, __VA_ARGS__);
> 	 kernel_fpu_end();
> 	 __s;
> 
> I'm not sure about the
> 
> 	WARN_ON_ONCE(!irq_fpu_usable());
> 
> thing in kernel_fpu_begin() though, I guess it wouldn't hurt...

Hmm... note that we may call EFI runtime services from interrupt context
in efi_pstore_write(), so it seems like it would be possible to trigger
that WARN_ON_ONCE() there.

Seiji (Cc'd) might have some opinions on this.

Either way, if someone sends me a patch ontop of this one that swaps the
__kernel_fpu_begin() for kernel_fpu_begin() I can try them out in my
lab.

-- 
Matt Fleming, Intel Open Source Technology Center
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