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

[Laurent Vivier]

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 <address@hidden> 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