[libvirt] [PATCH v4 2/5] libxl: add support for PVH

[Jim Fehlig]

On 10/19/18 8:14 AM, Daniel P. Berrangé wrote:
> On Fri, Oct 19, 2018 at 08:06:18AM -0600, Jim Fehlig wrote:
>> On 10/19/18 3:11 AM, Daniel P. Berrangé wrote:
>>> On Thu, Oct 18, 2018 at 11:08:34AM -0600, Jim Fehlig wrote:
>>>> On 10/17/18 12:59 PM, Marek Marczykowski-Górecki wrote:
>>>>> On Sat, Oct 13, 2018 at 08:46:19AM -0600, Jim Fehlig wrote:
>>>>>> I had some couch time at the start of the weekend and was finally able to
>>>>>> try using this series with virt-install. As it turns out, reporting
>>>>>> duplicate <guest> blocks for <os_type> 'xen' is not quite right. Instead we
>>>>>> will want to report the additional <machine> under the existing 'xen'
>>>>>> <guest> blocks.
>>>>>
>>>>> Is that virt-install limitation? In that case, IMO virt-install should
>>>>> be fixed, instead of changing capabilities xml to match its limitations.
>>>>
>>>> Perhaps it is a virt-install limitation, but my suggestion was based more on
>>>> how the qemu driver reports the different machines
>>>>
>>>> <guest>
>>>>     <os_type>hvm</os_type>
>>>>     <arch name='x86_64'>
>>>>       <wordsize>64</wordsize>
>>>>       <emulator>/usr/bin/qemu-system-x86_64</emulator>
>>>>       <machine maxCpus='255'>pc-i440fx-3.0</machine>
>>>>       <machine maxCpus='288'>pc-q35-3.0</machine>
>>>>       ...
>>>>     </arch>
>>>> </guest>
>>>>
>>>> Compare that with reporting PV and PVH in different <guest> blocks, where
>>>> the <os_type> and <arch> are the same. It seems confusing from a consumers
>>>> POV
>>>>
>>>> <guest>
>>>>     <os_type>xen</os_type>
>>>>     <arch name='x86_64'>
>>>>       <wordsize>64</wordsize>
>>>>       <emulator>/usr/bin/qemu-system-x86_64</emulator>
>>>>       <machine>xenpv</machine>
>>>>     </arch>
>>>> </guest>
>>>>
>>>> <guest>
>>>>     <os_type>xen</os_type>
>>>>     <arch name='x86_64'>
>>>>       <wordsize>64</wordsize>
>>>>       <emulator>/usr/bin/qemu-system-x86_64</emulator>
>>>>       <machine>xenpvh</machine>
>>>>     </arch>
>>>> </guest>
>>>>
>>>> How should a consumer interpret that? Is the machine for os_type=xen,
>>>> arch=x86_64 a xenpv or a xenpvh?
>>>
>>> Yes, you are right - any pair of (os_type, arch) should be unique
>>> in the capabilities XML. So all machines should be reported in the
>>> same block.
>>
>> Our difficulty with that is xenpv and xenpvh machines have different
>> features. Marek pointed out that the qemu driver reports the "feature"
>> maxCpus as an attribute on the machine element, but we're hesitant to keep
>> adding attributes for each feature that is unique to a machine.
>>
>> Another option we discussed was reporting a superset of all features so that
>> e.g. (xen, x86_64) block would contain features supported by both PV and PVH
>> and then rejecting unsupported features when parsing domXML or starting the
>> VM. This option is rather distasteful.
>>
>> And we also have the option of adding VIR_DOMAIN_OSTYPE_XENPVH, which I've
>> shied away from but may be a better way to go in the end. Do you have any
>> suggestions we may have overlooked?
> 
> Oooh, it looks like i've been mis-understanding PVH in all my previous
> reviews.
> 
> I thought it was simply a "normal" Xen paravirtualized guest kernel. ie
> any 'pv' guest is also a valid 'pvh' guest. Looking at the docs
> 
>    https://wiki.xen.org/wiki/Xen_Project_Software_Overview#Guest_Types
> 
> It appears I was wrong. It says a pvh guest kernel relies on hardware
> virt extensions for part of its work and paravirt for other parts. So
> really is a hybrid between pv and hvm.

Right. The Xen wiki also has a good writeup about the various guest types

https://wiki.xenproject.org/wiki/Understanding_the_Virtualization_Spectrum

> With that in mind, we should indeed have a distinct OS type constant
> to express this.

There have been some long threads in the various versions of this series with a 
lot of waffling :-). I made a few attempts at summarizing what we learned about 
PV vs PVH but could never build a strong case (at least in my own head) for 
either of the two modeling approaches

https://www.redhat.com/archives/libvir-list/2018-October/msg00214.html
https://www.redhat.com/archives/libvir-list/2018-October/msg00891.html

Regards,
Jim