From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Wed, 17 Dec 2014 18:17:34 +0100
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Message-ID: <20141217171734.GE2012@hermes.click-hack.org>
References: <5491B5E9.4030200@siemens.com>
 <5491BB1B.7020306@xenomai.org>
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Subject: Re: [Xenomai] Mayday mechanism broken on x86-64 - simpler approach
 feasible?
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To: Philippe Gerum <rpm@xenomai.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Xenomai <xenomai@xenomai.org>

On Wed, Dec 17, 2014 at 06:19:23PM +0100, Philippe Gerum wrote:
> On 12/17/2014 05:57 PM, Jan Kiszka wrote:
> > Hi,
> > 
> > while porting the sigdebug test case to forge I noticed occasional
> > crashes of the test after it received a watchdog signal. It turned out
> > that the problem is in the way mayday works on x86-64:
> > 
> > 1. sp/ip/ax of interrupted thread is saved
> > 2. ip is set to mayday page
> > 3. mayday page executes the mayday syscall via the syscall instruction
> > 4. the syscall restores state on original state on return
> > 
> > That that's the theory. Unfortunately the syscall instructions
> > overwrites the cx register with the caller's instruction pointer. We
> > could save it but there is no way to restore it on syscall return
> > without significant changes to the Linux code in entry_64.S.

Maybe we can restore it in xnarch_fixup_mayday, if nothing touches
it ?

-- 
					    Gilles.