Re: [Qemu-devel] virtio-rng and fd passing

[Anthony Liguori <anthony@codemonkey.ws>]

Stefan Berger <stefanb@linux.vnet.ibm.com> writes:

> On 03/01/2013 03:04 PM, Anthony Liguori wrote:
>> Eric Blake <eblake@redhat.com> writes:
>>
>>> Stefan Berger and I discovered on IRC that virtio-rng is unable to
>>> support fd passing.  We attempted:
>>>
>>> qemu-system-x86_64 ... -add-fd set=4,fd=34,opaque=RDONLY:/dev/urandom
>>> -object rng-random,id=rng0,filename=/dev/fdset/4 -device
>>> virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x6
>> Why are you using th rng-random backend instead of the rng-egd backend?
>
> I followed the example on the libvirt website trying out the XML for the 
> rng device while adpating the libvirt code to use fd passing and 
> generating the -add-fd command line parameters.
>
>>
>> You can pass chardevs to the egd backend.  It's really not a good idea
>> to pass a fd via rng-rangom.
>
> Fine, then we won't use fd passing for this device, whatever the reason 
> may be.

So let's step back.  There are two backends currently supported:
rng-random and rng-egd.  I don't see any point in taking an fd for
rng-random.  I don't think labeling comes into play here.

But if libvirt wants to interact with virtio-rng in a more intelligent
way (implementing a policy to distribute entropy), then rng-egd is the
right way to do that.

Regards,

Anthony Liguori

>
>     Stefan