Re: [Xen-devel] [Patch] cmpxchg emulation returns wrong ZF

To:	Jan Beulich <JBeulich@xxxxxxxxxx>
Subject:	Re: [Xen-devel] [Patch] cmpxchg emulation returns wrong ZF
From:	Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
Date:	Thu, 06 Aug 2009 10:12:24 +0200
Cc:	"xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Delivery-date:	Thu, 06 Aug 2009 01:12:53 -0700
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Jan Beulich wrote:
>>>> Juergen Gross <juergen.gross@xxxxxxxxxxxxxx> 06.08.09 08:49 >>>
>> Hi,
>>
>> attached patch corrects a bug in cmpxchg emulation in the hypervisor.
>>
>> BS2000 running as HVM-domain on 4 vcpus (no HAP) hit an error due to this bug
>> after several days.
> 
> Why don't you just clear ZF in that case? I think it is intentional that the
> code doesn't loop inside the hypervisor, since that loop is non-preemptible
> (whereas returning to the guest and re-issuing the instruction is).
> 
> Further, I'm not really clear why that change is necessary at all: In the
> code prior to the patch, register state is not being updated if
> ops->cmpxchg() failed, and hence the old value of ZF is simply being
> retained - which is the correct thing to do when intending to re-start
> the instruction.

Oh yes, you are right!
I missed that eip isn't updated then, too.

Please forget that patch. I'll continue to investigate the problem...


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 636 47950
Fujitsu Technolgy Solutions               e-mail: juergen.gross@xxxxxxxxxxxxxx
Otto-Hahn-Ring 6                        Internet: ts.fujitsu.com
D-81739 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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Re: [Xen-devel] [Patch] cmpxchg emulation returns wrong ZF