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Date:	Sat, 1 Feb 2014 18:05:05 -0800
From:	Linus Torvalds <>
To:	"H. Peter Anvin" <>
Cc:	Suresh Siddha <>,
	Nate Eldredge <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	"the arch/x86 maintainers" <>,
	stable <>,
	Linux Kernel Mailing List <>,
	Maarten Baert <>,
	Jan Kara <>, George Spelvin <>,
	Pekka Riikonen <>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

On Sat, Feb 1, 2014 at 5:57 PM, H. Peter Anvin <> wrote:
> Twiddling CR0.TS is pretty slow if we're not taking advantage of it.


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.

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