linux-kernel - Re: X86 fpu registers in a signal handler's ucontext

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Message-ID: <51832C6C.1030805@zytor.com>
Date:	Thu, 02 May 2013 20:18:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Warlich, Christof" <christof.warlich@...mens.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: X86 fpu registers in a signal handler's ucontext

On 04/29/2013 07:34 AM, Warlich, Christof wrote:
> Second, only FP state seems to be available through ucontext_t, and I would
> need to clear exceptions for SSE as well.

Not true.

> Can anyone give me some advice on how to I could proceed?

To optimize the common case, if the signal handler doesn't touch the FPU
the state is not reloaded from the signal stack.  Therefore you need to
execute an FPU instruction -- pretty much any FPU instruction -- in your
signal handler in order to force the reload.

	-hpa

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