[libvirt] [PATCH 3/3] qemu_process: add check for hyperv features

[Maxim Nestratov]

30.03.2016 10:50, Pavel Hrdina пишет:
> On Tue, Mar 29, 2016 at 07:06:14PM +0300, Maxim Nestratov wrote:
>> 29.03.2016 16:31, Pavel Hrdina пишет:
>>> Commit 7068b56c introduced several hyperv features.  Not all hyperv
>>> features are supported by old enough kernels and we shouldn't allow to
>>> start a guest if kernel doesn't support any of the hyperv feature.
>>>
>>> There is one exception, for backward compatibility we cannot error out
>>> if one of the RELAXED, VAPIC or SPINLOCKS isn't supported, for the same
>>> reason we ignore invtsc, to not break restoring saved domains with older
>>> libvirt.
>>>
> [...]
>
>> Hmm, qemu already checks them and simply ignores most of them and
>> doesn't prevent guest from starting in case they are not supported and
>> optional. In case they are reqired it fails. Why should we check them
>> here? At least we should follow the logic qemu has.
> Yes, that's true that QEMU do some checks and ignores most of the features
> missing in kernel, but that's no reason why we should do the same.  Libvirt
> tries to present in domain XML only those features and devices that are actually
> present in the guest.  Thus if you tell libvirt that you want some hyperv
> feature but your host kernel doesn't support it, we should let the user know
> that this feature isn't supported instead of ignoring that fact and start the
> guest anyway.
>
> Pavel
I see your point. And what if a user wants to define some features just 
to ask libvirt/qemu to do its best if possible and ignore if it isn't 
without redefining domain xml? Does such case make sense?

Maxim