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Date: Sat, 1 Feb 2014 17:59:00 -0800
Message-ID: <CALmL7E8eXidXSVW2wsvjYZApcdYCYUbzqc_tHOkboB1qS_ngiw@mail.gmail.com>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag
From: Suresh Siddha <sbsiddha@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.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>
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Sender: linux-kernel-owner@vger.kernel.org

On Sat, Feb 1, 2014 at 5:51 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Sat, Feb 1, 2014 at 5:47 PM, Suresh Siddha <sbsiddha@gmail.com> wrote:
>>
>> So if the restore failed, we should do something like drop_init_fpu(),
>> which will restore init-state to the registers.
>>
>> for eager-fpu()  paths we don't use clts() stts() etc.
>
> Uhhuh. Ok.
>
> Why do we do that, btw? I think it would make much more sense to just
> do what I *thought* we did, and just make it a context-switch-time
> optimization ("let's always switch FP state"), not make it a huge
> semantic difference.

clts/stts is more costly and not all the state under xsave adhers to
cr0.TS/DNA rules.

did I answer your question?

thanks,
suresh
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