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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v2 1/1] virtio-rng: hardware random number generator device
Date: Mon, 04 Jun 2012 19:04:41 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

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

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

Invocation is simple:

   $ qemu ... -device virtio-rng-pci,chardev=foo

In the guest, we see

   $ cat /sys/devices/virtual/misc/hw_random/rng_available

   $ cat /sys/devices/virtual/misc/hw_random/rng_current

   # 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

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

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.


Anthony Liguori


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