linux-kernel - Re: [RFC] x86: Disable traditional FPU instructions too

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Date:	Tue, 28 Jan 2014 11:33:03 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Kosina <jkosina@...e.cz>, Michael Matz <matz@...e.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [RFC] x86: Disable traditional FPU instructions too

On Tue 28-01-14 02:25:09, H. Peter Anvin wrote:
> If that is intentional it needs a big huge freaking comment recording
> why it is not a bug. 

Agreed.

> I would expect that to generate a FP multiply.

Exactly! And actually the very same construct in a module provided to
reproduce an issue was a trigger to the discussed patch.

> On January 28, 2014 1:37:01 AM PST, Michal Hocko <mhocko@...e.cz> wrote:
> >[CCing Ralf for MIPS.]
> >
> >On Mon 27-01-14 17:45:44, Borislav Petkov wrote:
> >> On Mon, Jan 27, 2014 at 08:41:09AM -0800, H. Peter Anvin wrote:
> >> > I don't think it'd hurt... although I think the above pretty much
> >> > requires that the code contain actual floating-point types to ever
> >be
> >> > generated.
> >> 
> >> Yes, that is supposed to catch usage of the 'E' floating constant,
> >for
> >> example, in definitions. It is a build-time assertion, if you want.
> >:)
> >
> >And git grep says that mips abuses this notation:
> >arch/mips/include/asm/kvm_host.h:#define MS_TO_NS(x) (x * 1E6L)
> >
> >I do not have cross-compiler set up to check whether this is a real
> >issue because include/media/rc-core.h defines the same macro correctly
> >so the real code might end up using the right variant.
> 
> -- 
> Sent from my mobile phone.  Please pardon brevity and lack of formatting.

-- 
Michal Hocko
SUSE Labs
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