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

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Message-ID: <0b88762d-f9f9-419b-9c3e-a18f5458471c@email.android.com>
Date:	Sat, 01 Feb 2014 13:36:54 -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

It would be good to know how often #3 happens on a real workload.

On February 1, 2014 1:17:10 PM PST, George Spelvin <linux@...izon.com> 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).

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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