Re: [Qemu-devel] [PATCH v2 1/1] virtio-rng: hardware random number generator device

[Anthony Liguori]

On 05/26/2012 04:20 AM, Amit Shah wrote:
> On (Fri) 25 May 2012 [15:00:53], Anthony Liguori wrote:
> > On 05/25/2012 02:32 PM, Amit Shah wrote:
> > > The Linux kernel already has a virtio-rng driver, this is the device
> > > implementation.
> > >
> > > When the guest asks for entropy from the virtio hwrng, it puts a buffer
> > > in the vq.  We then put entropy into that buffer, and push it back to
> > > the guest.
> > >
> > > The chardev connected to this device is fed the data to be sent to the
> > > guest.
> > >
> > > Invocation is simple:
> > >
> > >    $ qemu ... -device virtio-rng-pci,chardev=foo
> > >
> > > In the guest, we see
> > >
> > >    $ cat /sys/devices/virtual/misc/hw_random/rng_available
> > >    virtio
> > >
> > >    $ cat /sys/devices/virtual/misc/hw_random/rng_current
> > >    virtio
> > >
> > >    # cat /dev/hwrng
> > >
> > > Simply feeding /dev/urandom from the host to the chardev is sufficient:
> > >
> > >    $ qemu ... -chardev socket,path=/tmp/foo,server,nowait,id=foo \
> > >               -device virtio-rng,chardev=foo
> > >
> > >    $ nc -U /tmp/foo<   /dev/urandom
> > >
> > > A QMP event is sent for interested apps to monitor activity and send the
> > > appropriate number of bytes that get asked by the guest:
> > >
> > >    {"timestamp": {"seconds": 1337966878, "microseconds": 517009}, \
> > >     "event": "ENTROPY_NEEDED", "data": {"bytes": 64}}
> >
> > I don't understand the point of this event.  Can't a management app
> > just create a socket and then it can see all the requests the guest
> > makes?
>
> How?  With the chardev, it can only keep feeding data, and that data
> will be consumed when chr_can_read() returns>  0.  And even then the
> mgmt app has no idea how much data was asked for, and how much was
> consumed.

Okay, then the right approach is to use a message protocol where QEMU asks for a 
certain amount of data and then the daemon sends it back.

I think this is pretty much why the egd protocol exists, no?  Why not just 
implement egd protocol support?

Once we introduce a protocol of any form (even raw), we have to support it 
forever so let's not do it carelessly.

Regards,

Anthony Liguori

>                 Amit