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

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

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On Mon, 21 Sep 2015 10:26:52 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On 21/09/15 10:01, Greg Kurz wrote:
> > On Mon, 21 Sep 2015 12:10:00 +1000
> > David Gibson <david@gibson.dropbear.id.au> 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 <thuth@redhat.com> 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?
> 

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.

> > Back to Thomas' patch, it does the job and brings H_RANDOM, which is
> > currently missing.
> > 
> > Acked-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > 
> > I could test both use-kvm and backend flavors (including migration).
> > 
> > Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> 
> Thanks!
> 
>  Thomas
> 
> 


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