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

[Thomas Huth <thuth@redhat.com>]

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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?

> 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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