[Xen-devel] Re: [PATCH] qemu-xen: Fix PV segfault

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Kevin Wolf writes ("Re: [PATCH] qemu-xen: Fix PV segfault"):
> Ian Jackson schrieb:
> > Kevin Wolf writes ("[PATCH] qemu-xen: Fix PV segfault"):
> >> Don't access cur_cpu with CONFIG_DM, for PV machines it is NULL.
> > 
> > I'm not sure I understand the execution context here properly.  Is
> > this for blktap ?  It would be better to make cur_cpu be set to some
> > sensible value.
> 
> No, this is completely independent of blktap. Currently, PV machines
> just don't have a virtual cpu in qemu - in contrary to FV machines which
> call a complete pc_machine.init() during their initialization.

Evidently this is a configuration I haven't tested.  My PV machines
don't use a qemu-dm at all.

> Maybe you're right and it would be better to assign even PV machines a
> cpu, but I'm not sure what other functionality this implies to
> implement. I'll try to look into this.

This isn't very hard, surely ?  The problem is presumably that the PV
startup is bypassing some of the necessary initialisation.  I
encountered this myself for the HVM case.

It is quite fine for qemu to have what it thinks of as a real CPU but
which isn't really used.  That's how the HVM guests work anyway.  All
that's needed is enough of a cheap plastic imitation that these kind
of enquiry calls (flag checks, etc.) don't die and give plausible
answers.

I would very much prefer an answer which didn't make any additional
changes to vl.c (or other code shared with upstream).  If such changes
are necessary they should be textually small.  This will make keeping
us up to date with qemu upstream much easier.

Ian.

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