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

[Eric Blake]

On 09/21/2015 12:00 AM, Thomas Huth wrote:

>>> 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.
> 
> I thought about this question on the weekend and came to the same
> conclusion. I think if we want to enable this by default, it likely
> should rather be done at the libvirt level instead?
> 

Adding hardware by default to existing machine types has been a bane to
libvirt usage in the past.  If upgrading from an old qemu to a new one
suddenly turns on new guest-visible hardware with no change to the
command line, then libvirt has a much harder time migrating that guest.
 It's okay to have a new machine type turn on a feature by default, and
to have knobs so that the feature can be turned on even for older
machine types, but experience has shown that any new feature MUST come
with knobs and a way to learn if the feature can be turned on/off,
rather than just blindly assuming that turning it on is the right thing.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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