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

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 :-(