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

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Message-ID: <20140201211710.20281.qmail@science.horizon.com>
Date:	1 Feb 2014 16:17:10 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	hpa@...or.com, torvalds@...ux-foundation.org
Cc:	jack@...e.cz, linux-kernel@...r.kernel.org, linux@...izon.com,
	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

> .. 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).
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