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

[Anthony Liguori <anthony@codemonkey.ws>]

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
>
>