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

[Amit Shah]

On (Mon) 04 Jun 2012 [19:04:41], Anthony Liguori wrote:
> 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.

Why is a message protocol necessary?  And a daemon isn't necessary,
too.

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

Perhaps I'm not getting what you're saying.

What we have here, via the event, is to tell the mgmt app that the
guest has need for entropy.  The guest also tells us exactly how many
bytes it needs.  We're just passing on that info to the mgmt app, if
it needs it.

In any case, any data pushed down the chardev will be consumed by the
device, and sent to the guest when the guest puts a buf in the vq.

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

There's no need for a protocol at all here.  Can you skim through the
code and point out where you think we're disagreeing?

                Amit