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

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Message-ID: <4b084999-5ea6-4fd5-b6e9-c9066f4bb55b@email.android.com>
Date:	Sat, 01 Feb 2014 18:12:18 -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

Yes, that is exactly the "eageronly" features - currently LWP and MPX.

On February 1, 2014 6:05:05 PM PST, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sat, Feb 1, 2014 at 5:57 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> Twiddling CR0.TS is pretty slow if we're not taking advantage of it.
>
>Immaterial.
>
>We *already* avoid twiddling TS if it's not needed.
>
>It is true that we used to twiddle it at every context switch (and
>then twiddle it *again* if we decided that we'd want to pre-load the
>FPU state anyway, and avoid the extra fault).
>
>But that was fixed, and if we switch from a task that had math state
>to another task that has math state, we leave TS alone.
>
>But Suresh apparently hits on the real issue:
>
>> not all the state under xsave adhers to cr0.TS/DNA rules
>
>which if so is sad but yes, makes CR0.TS no longer sufficient.
>
>             Linus

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