Re: [Qemu-devel] [PATCH v2] rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

[Laurent Vivier <lvivier@redhat.com>]

On 10/05/2019 17:32, Daniel P. Berrangé wrote:
> On Fri, May 10, 2019 at 02:37:41PM +0200, Laurent Vivier wrote:
>> On 10/05/2019 14:27, Markus Armbruster wrote:
>>> Laurent Vivier <lvivier@redhat.com> writes:
>>> The new rng-builtin is considerably simpler than both rng-random and
>>> rng-egd.  Moreover, it just works, whereas rng-random is limited to
>>> CONFIG_POSIX, and rng-egd needs egd running (which I suspect basically
>>> nobody does).  Have we considered deprecating these two backends in
>>> favor of rng-builtin?
>>
>> I have several bugzilla involving these backends: as there are blocking, the
>> virtio-rng device in the guest can hang, or crash during hot-unplug. From my
>> point of view, life would be easier without them...
> 
> Are you sure about that ?
> 
> The EGD impl looks like it is requesting entropy in an async manner.

The virtio-rng driver waits until it receives enough entropy from the 
RNG backend while a mutex is taken.

If the EGD daemon doesn't provide enough data to the RNG backend, 
virtio-rng driver can hang.

It's easy to have if we start EGD backend with a socket in server,nowait 
mode and no EGD daemon connects to the port.

Thanks,
Laurent