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Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Sat, 01 Feb 2014 18:12:18 -0800
To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Suresh Siddha <sbsiddha@gmail.com>,
        Nate Eldredge <nate@thatsmathematics.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        stable <stable@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Maarten Baert <maarten-baert@hotmail.com>, Jan Kara <jack@suse.cz>,
        George Spelvin <linux@horizon.com>, Pekka Riikonen <priikone@iki.fi>
Message-ID: <4b084999-5ea6-4fd5-b6e9-c9066f4bb55b@email.android.com>
Sender: linux-kernel-owner@vger.kernel.org

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

On February 1, 2014 6:05:05 PM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Sat, Feb 1, 2014 at 5:57 PM, H. Peter Anvin <hpa@zytor.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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