Re: [Qemu-devel] [PATCH v4] ppc/spapr: Implement H_RANDOM hypercall in QEMU

[David Gibson]

On Mon, Sep 21, 2015 at 10:37:28AM +0200, Greg Kurz wrote:
> On Mon, 21 Sep 2015 10:26:52 +0200
> Thomas Huth <address@hidden> wrote:
> 
> > On 21/09/15 10:01, Greg Kurz wrote:
> > > On Mon, 21 Sep 2015 12:10:00 +1000
> > > David Gibson <address@hidden> wrote:
> > > 
> > >> On Fri, Sep 18, 2015 at 11:05:52AM +0200, Greg Kurz wrote:
> > >>> On Thu, 17 Sep 2015 10:49:41 +0200
> > >>> Thomas Huth <address@hidden> wrote:
> > >>>
> > >>>> The PAPR interface defines a hypercall to pass high-quality
> > >>>> hardware generated random numbers to guests. Recent kernels can
> > >>>> already provide this hypercall to the guest if the right hardware
> > >>>> random number generator is available. But in case the user wants
> > >>>> to use another source like EGD, or QEMU is running with an older
> > >>>> kernel, we should also have this call in QEMU, so that guests that
> > >>>> do not support virtio-rng yet can get good random numbers, too.
> > >>>>
> > >>>> This patch now adds a new pseudo-device to QEMU that either
> > >>>> directly provides this hypercall to the guest or is able to
> > >>>> enable the in-kernel hypercall if available.
> > ...
> > >>> It is a good thing that the user can choose between in-kernel and 
> > >>> backend,
> > >>> and this patch does the work.
> > >>>
> > >>> This being said, I am not sure about the use case where a user has a 
> > >>> hwrng
> > >>> capable platform and wants to run guests without any hwrng support at 
> > >>> all is
> > >>> an appropriate default behavior... I guess we will find more users that 
> > >>> want
> > >>> in-kernel being the default if it is available.
> > >>>
> > >>> The patch below modifies yours to do just this: the pseudo-device is 
> > >>> only
> > >>> created if hwrng is present and not already created.
> > >>
> > >> I have mixed feelings about this.  On the one hand, I agree that it
> > >> would be nice to allow H_RANDOM support by default.  On the other hand
> > >> the patch below leaves no way to turn it off for testing purposes.  It
> > >> also adds another place where the guest hardware depends on the host
> > >> configuration, which adds to the already substantial mess of ensuring
> > >> that source and destination hardware configuration matches for
> > >> migration.
> > > 
> > > Yeah, describing the guest hw is really essential for migration... this
> > > is best addressed at the libvirt level with a full XML description of
> > > the machine... but FWIW if we are talking about running pseries on a
> > > POWER8 or newer host, I am not aware about "hwrng-less" boards... but
> > > I am probably missing something :)
> > 
> > Maybe it would be at least ok to enable it by default as long as
> > "-nodefaults" has not been specified as command line option?

I like that in principle, but the -nodefaults option isn't exposed
outside vl.c

> It makes a lot of sense indeed. I guess David should take your patch
> as it is now and the default behavior could be a follow up.

That's the plan.  I've already  taken the base patch.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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