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

[Daniel P. Berrange]

On Sat, May 26, 2012 at 01:02:49AM +0530, 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

ACK to this ARGV design from a libvirt POV.

> 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}}

IIUC, there are three ways mgmt apps can use the RNG with the
chardev

 - Wire it up to a source that just blindly provides all the
   entropy QEMU desires (as you /dev/urandom example above)

 - Feed in a fixed amount of entropy every minute, regardless
   of how much QEMU desires

 - Feed in entropy on demand, in response to the ENTROPY_NEEDED
   event notification (possibly throttling)

These options sounds like they should cover all reasonable needs,
so gets my vote. Probably want to include the ENTROPY_NEEDED
event in my patch which adds rate limiting to guest initiated
events.

Daniel
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