linux-kernel - Re: [PATCH REPOST] Run IST traps from user mode preemptive on process stack

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Message-ID: <20080311132616.GH18917@one.firstfloor.org>
Date:	Tue, 11 Mar 2008 14:26:16 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Zdenek Kabelac <zdenek.kabelac@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	tglx@...utronix.de, linux-kernel@...r.kernel.org, jkosina@...e.cz
Subject: Re: [PATCH REPOST] Run IST traps from user mode preemptive on process stack

> I'm not sure whether this is good or bad sign - but with this patch
> you have posted,
> I do not have to wait in qemu for minutes to get the 'busy-loop' - I
> get the exact same loop almost instantly when I start  disk read of LV
> partition and running my simple module insertion testcase.

Hmm, my fix was intended to fix a gdb problem. Or rather the gdb
problem was already fixed by disabling some functionality and with
this patch the functionality would be reenabled again.

If it fixed a qemu problem too that's great but was unintended
on my part. In fact it is a little worrying because there should
be no user visible change. Can you double check by unapply/test/reapply/test
the patch really makes a difference? 

> 
> >  This patch changes these traps instead to always switch
> >  to the process stack when the trap originated from user mode.
> >  For kernel traps it keeps running non preemptive on the IST stack
> >   because that is much safer (e.g. to still get nmi watchdog events
> >  out even when the process stack is corrupted)
> >
> 
> So what should I check to get fixed my problem in qemu ?

I don't know. Someone has to debug it.

-Andi
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