On 21/09/15 10:01, Greg Kurz wrote: > On Mon, 21 Sep 2015 12:10:00 +1000 > David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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? > Back to Thomas' patch, it does the job and brings H_RANDOM, which is > currently missing. > > Acked-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> > > I could test both use-kvm and backend flavors (including migration). > > Tested-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> Thanks! Thomas
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