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

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Date:	Sat, 01 Feb 2014 15:40:13 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	George Spelvin <linux@...izon.com>, torvalds@...ux-foundation.org
CC:	jack@...e.cz, linux-kernel@...r.kernel.org,
	maarten-baert@...mail.com, mingo@...nel.org,
	nate@...tsmathematics.com, priikone@....fi, sbsiddha@...il.com,
	stable@...r.kernel.org, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt
 flag

On 02/01/2014 01:17 PM, George Spelvin wrote:
>> .. 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.
> 
> This definitely seems much more sensible.  For processors without
> optimized context save/restore, the decision whether to restore eagerly
> is best made once the kernel has decided what it's returning *to*.
> 
> From an interrupt handler, there are now three cases:
> 1) FPU not in use, available for immediate use.
> 2) FPU in use by user-space; save to <location> before using.
> 3) FPU in use by kernel code; may not be used (unless we want
>    to add nested FPU state save hair).
> 

OK, let's circle back for a bit.  We have an active bug, and we clearly
have a lot of restructuring that could/should be done.  We need to fix
the bug first; if we're going to a bunch of restructuring then that
ought to be separate.  The first bit is how we fix the immediate bug.

	-hpa

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