Xen devel - Apr 2008 - [PATCH] create proper net device for hvm guests

I''ve noticed that hvm guests using pv network device also get an
emulated network device.  Conversely, hvm guests using emulated network
device also get pv network device.  I''m surprised that this
hasn''t been
fixed already, which makes me think there is some unforeseen issue that
results from not providing both.  I guess one drawback is that the guest
config file must be edited to remove ''model=foo, type=ioemu''
when using
pv network device  - but it seems the guest config file should be the
place where these types of settings are explicitly stated.

I''ve attached a patch the creates either the emulated or pv network
device (but not both) depending on setting in guest configuation, i.e.
''type=ioemu''.  Please apply if my reasoning is sane.  If not,
please
clarify reason for presenting both :-).

Cheers,
Jim


Create either the emulated or pv network device (but not both) depending
on setting in guest configuation, i.e. ''type=ioemu''.

    Signed-off-by: Jim Fehlig <jfehlig@novell.com>




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On Fri, Apr 04, 2008 at 05:25:57PM -0600, Jim Fehlig
wrote:
> I''ve noticed that hvm guests using pv network device also get an
> emulated network device.  Conversely, hvm guests using emulated network
> device also get pv network device.  I''m surprised that this
hasn''t been
> fixed already, which makes me think there is some unforeseen issue that
> results from not providing both.  I guess one drawback is that the guest
> config file must be edited to remove ''model=foo,
type=ioemu'' when using
> pv network device  - but it seems the guest config file should be the
> place where these types of settings are explicitly stated.
> 
> I''ve attached a patch the creates either the emulated or pv
network
> device (but not both) depending on setting in guest configuation, i.e.
> ''type=ioemu''.  Please apply if my reasoning is sane.  If
not, please
> clarify reason for presenting both :-).
The guest administrator does not have control over the Dom0, and thus has
no ability to change the config. Since installing Xen PV drivers is a post
install task, you can''t create the guest with pv-only mode
straightaway,
and nor can the guest admin change the config. Thus the presenting both the
devices lets the guest admin choose between the two at will. They can
easily blacklist the ''8139too'' driver in modprobe.conf if they
only want
to use the pv driver and prevent the 2nd nic from being mistakenly used.

IIRC, the logic is supposed to currently be:

  - type=ioemu  - only present emulated rtl8139 (or as per model=)
  - type=netfront - only present the pv driver
  - no type= setting - present both pv & rtl8139 nic

Regards,
Dan.
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Daniel P. Berrange wrote:
> On Fri, Apr 04, 2008 at 05:25:57PM -0600, Jim Fehlig wrote:
>   
>> I''ve noticed that hvm guests using pv network device also get
an
>> emulated network device.  Conversely, hvm guests using emulated network
>> device also get pv network device.  I''m surprised that this
hasn''t been
>> fixed already, which makes me think there is some unforeseen issue that
>> results from not providing both.  I guess one drawback is that the
guest
>> config file must be edited to remove ''model=foo,
type=ioemu'' when using
>> pv network device  - but it seems the guest config file should be the
>> place where these types of settings are explicitly stated.
>>
>> I''ve attached a patch the creates either the emulated or pv
network
>> device (but not both) depending on setting in guest configuation, i.e.
>> ''type=ioemu''.  Please apply if my reasoning is sane. 
If not, please
>> clarify reason for presenting both :-).
>>     
>
> The guest administrator does not have control over the Dom0, and thus has
> no ability to change the config. Since installing Xen PV drivers is a post
> install task, you can''t create the guest with pv-only mode
straightaway,
> and nor can the guest admin change the config.
Yes, that was the drawback I noted above.
>  Thus the presenting both the
> devices lets the guest admin choose between the two at will. They can
> easily blacklist the ''8139too'' driver in modprobe.conf if
they only want
> to use the pv driver and prevent the 2nd nic from being mistakenly used.
>
> IIRC, the logic is supposed to currently be:
>
>   - type=ioemu  - only present emulated rtl8139 (or as per model=)
>   - type=netfront - only present the pv driver
>   - no type= setting - present both pv & rtl8139 nic
>   
On 3.2 I observe

  - type=ioemu - both emulated and pv presented
  - type=netfront - only pv presented
  - no type setting - both emulated and pv presented

The second hunk of this patch provides the behavior you described. 
However, when using it, I noticed that no vif entries are created in
xenstore frontend/backend paths (DevController.createDevice is not
called when ''type=ioemu'').  Oddly, I don''t see any
ill affects from that
though ;-).

The first hunk changes

  -no type setting - both emulated and pv presented

to

-no type setting - only pv presented

so ignore that.  I would suspect that many guests targeted for pv driver
installation have ''type=ioemu'' in their config, making the
second hunk
of this patch distasteful as well.  Sigh ...

Jim

> Regards,
> Dan.
>   
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On 5 Apr 2008, at 3:21am, Jim Fehlig wrote:
> However, when using it, I noticed that no vif entries are created in
> xenstore frontend/backend paths (DevController.createDevice is not
> called when ''type=ioemu'').  Oddly, I don''t see
any ill affects from
> that
> though ;-).
This would break Solaris dom0, which relies on the existence of the  
backend paths in the ioemu case.


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