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

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> 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.

Jan


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