Date: Thu, 7 Dec 2000 19:04:12 +0100 (MET)
From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>
To: Petr Vandrovec <VANDROVE@vc.cvut.cz>
cc: "Richard B. Johnson" <root@chaos.analogic.com>, richardj_moore@uk.ibm.com,
        linux-kernel@vger.kernel.org
Subject: Re: Why is double_fault serviced by a trap gate?
In-Reply-To: <F00C8C3408E@vcnet.vc.cvut.cz>
Message-ID: <Pine.GSO.3.96.1001207185903.21086J-100000@delta.ds2.pg.gda.pl>
Organization: Technical University of Gdansk
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, 7 Dec 2000, Petr Vandrovec wrote:

> No. If interrupt uses task gate, task switch happens. Nothing is stored
> in context of old process except registers into TSS. There is only one
> (bad) problem. If you want to get it 100% proof (it is not needed for double
> fault, but it is definitely needed for NMI, as NMI is very often on SMP
> ia32), each CPU's IRQ vector must point to different task, otherwise you
> can get TSS in use during doublefault, leading to triplefault again...

 Well, I expect wasting a descriptor and a page of memory for the purpose
of a TSS is not a big problem.

> Yes. Currently if any ESP related problem happens in kernel, machine silently
> reboots without any message. With task gate (as Jeff Merkey proposed

 You might handle the stack fault with a task gate, actually, but I'm not
sure it's worth the hassle.  Handling just the double fault should be
sufficient. 

> some months ago, btw), you can even suspend offending task and recover
> from it... I think that also bluesmoke should use task gate, but I
> did not read documentation on this yet.

 Yep.  An MCE is an abort like a double-fault, so the CPU state might be
corrupted (by definition -- I have no idea whether it really happens). 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

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