linux-kernel - Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

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Message-ID: <74304a13-a3b1-451c-870e-034b66224dc4@email.android.com>
Date:	Sat, 01 Feb 2014 12:00:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Suresh Siddha <sbsiddha@...il.com>,
	Nate Eldredge <nate@...tsmathematics.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	stable <stable@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maarten Baert <maarten-baert@...mail.com>,
	Jan Kara <jack@...e.cz>, George Spelvin <linux@...izon.com>,
	Pekka Riikonen <priikone@....fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

Of course, if we are really doing all eager fpu, we could just leave cr0.ts always clear and not touch it at all...

On February 1, 2014 11:46:15 AM PST, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sat, Feb 1, 2014 at 11:35 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> This will obviously not protect eageronly features (MPX, LWP, ...) so
>this
>> means those features are permanently unavailable to the kernel, even
>inside
>> kernel_fpu_begin/end.  Now, currently I don't think we have any plans
>to use
>> those in the kernel (at least not in a way where kernel_fpu_begin/end
>makes
>> sense as bracketing), but it is something worth keeping in mind.
>
>Hmm. If there are features that (silently) depend on the FPU state
>being on, then the plain "stts" doesn't work at all, because we'd be
>returning to user space too (not just kernel space) with TS turned
>off.
>
>.. which *does* actually bring up something that might work, and might
>be a good idea: remove the "restore math state or set TS" from the
>normal kernel paths *entirely*, and move it to the "return to user
>space" phase.
>
>We could do that with the whole "task_work" thing (or perhaps just
>do_notify_resume(), especially after merging the "don't necessarily
>return with iret" patch I sent out earlier), with additionally making
>sure that scheduling does the right thing wrt a "currently dirty math
>state due to kernel use".
>
>The advantage of that would be that we really could do a *lot* of FP
>math very cheaply in the kernel, because we'd pay the overhead of
>kernel_fpu_begin/end() just once (well, the "end" part would be just
>setting the bit that we now have dirty state, the cost would be in the
>return-to-user-space-and-restore-fp-state part).
>
>Comments? That would be much more invasive than just changing
>__kernel_fpu_end(), but would bring in possibly quite noticeable
>advantages under loads that use the FP/vector resources in the kernel.
>
>                Linus

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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