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

Fri Oct 19 14:59:03 UTC 2018

On Fri, Oct 19, 2018 at 08:53:15AM -0600, Jim Fehlig wrote:
> 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

It has a bad name, but essentially you should consider "ostype" to
refer to the   Hypervisor <-> Guest hardware ABI.

Based on what I read, and your 2 links here, PV is clearly a different
hardware ABI from PVH. Guest kernels needs different modifications for
PV vs PVH.

Sorry I didn't spot this sooner, and let this go off down the blind
alley of switching based on machine type, when we should have used
the ostype :-(

Regards,
Daniel
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