linux-kernel - Re: [PATCH] i386: improve double fault handling

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Message-ID: <48888289.4040001@zytor.com>
Date:	Thu, 24 Jul 2008 09:24:25 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Joerg Roedel <joro@...tes.org>, mingo@...e.hu,
	Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386: improve double fault handling

Jan Beulich wrote:
>>>> Joerg Roedel <joro@...tes.org> 23.07.08 23:43 >>>
>> On Fri, Jul 18, 2008 at 01:30:42PM +0100, Jan Beulich wrote:
>>> Make the double fault handler use CPU-specific stacks. Add some
>>> abstraction to simplify future change of other exception handlers to go
>>> through task gates.
>> What is the benefit of exception handlers going through task gates?
>> Hardware task switches are not very well supported in virtualization
>> (e.g. its has issues in KVM and is also not in Xen for a long time).
> 
> The main goal is to get to a different stack. While at present this is done
> only for the double fault, I think generally NMI and MCE should also do
> so, as they may be caused by a stack access (see x86-64, which runs
> them on IST stacks), and hence continuing to run on that same stack
> may not allow the exception to be handled.

NMI, MCE and #DF are the obvious candidates.

Now, keep in mind TSSes have to be prepared per-CPU, since they get 
marked "busy" when in use, so it's a bit of a nontrivial undertaking.

	-hpa

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