[Xen-devel] Re: [Qemu-devel] [PATCH 05/13] xen: groundwork for xen support

[Gerd Hoffmann <kraxel@xxxxxxxxxx>]

Ian Jackson wrote:
> Gerd Hoffmann writes ("[Qemu-devel] [PATCH 05/13] xen: groundwork for xen 
> support"):
>> +/*
>> + * Figure the environment we are running in.
>> + * Returns true when xen is present, false otherwise.
>> + * Also checks whenever the domain specified via -domid
>> + * exists (so we can attach) or whenever it must be created.
>> + */
> 
> I don't think this is the right approach.  The intent appears to be
> that if you run a particular qemu rune, it will do a completely
> different thing when running under Xen.

That is the intended default behaviour, yes.  If you run on Xen, you
most likely want to use it, right?  That matches behaviour of kvm and
kqemu:  If support is available, it is used by default (there are
options to turn it off).

> One bad effect of this is that it is not possible to do a software
> emulation of the Xen environment if the host happens to actually be
> running under Xen.

That isn't a fundamental issue.  It can trivially be fixed by adding a
command line option, which just has to set xen_present = 0.

> The command line, or chosen qemu executable, should specify what qemu
> is supposed to do:
>   - supporting tasks (hardware emulation etc.)
>     for a real Xen domain managed by Xen tools via xenstore etc.
>   - software emulation of the Xen environment for the benefit
>     of a Xen domU but without using the Xen hypervisor (and
>     this should obviously work even if running under Xen0
>   - whatever other use cases are important; apparently Gerd
>     seems keen to run some code with parts of the xend toolstack
>     replaced

We can certainly add command line options to force a specific operation
mode, see above.  I don't think such an option should be required
though.  IMHO qemu can and should pick sensible defaults.

>> +           "-domid          specify xen guest domain id\n"
> 
> Now would be a good time to rename this option to have the word `xen'
> in it.

How about -xen-domid?  And prefix any other xen specific options with
-xen too of course?

cheers,
  Gerd

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