[ovirt-devel] rethinking fake-kvm and faqemu

[Dan Kenigsberg]

On Mon, Oct 05, 2015 at 02:53:00PM +0200, Martin Polednik wrote:
> On 05/10/15 11:31 +0200, Michal Skrivanek wrote:
> >
> >On Oct 3, 2015, at 20:48 , Martin Polednik <mpolednik at redhat.com> wrote:
> >
> >>Hello everyone,
> >>
> >>I've been reworking the fake_kvm and faqemu VDSM hook to make them
> >>somewhat more usable and mostly to allow testing of ppc64le on x86_64
> >>based hosts.
> >>
> >>TL;DR version: checkout [1], enable fake_kvm and happy ppc64le hacking :)

I like the initiative and the approach. Dropping faqemu bits out of
mainline caps.py was an old task of mine. Moving code into lib/vdsm is a
big task for our next release anyway.

> >>
> >>Current fake_kvm isn't really a hook and is contained within 'caps'
> >>module. This is wrong for multiple reasons, the most important one
> >>being mixing optional with mainline code. Another issue appears when
> >>one tries to move the fake_kvm code into a hook: the whole notion of
> >>architectures within VDSM is contained within the 'caps' module.
> >>
> >>The patch series, which git tip is at [1], introduces new cpuinfo
> >>module that moves information related to architecture to 'cpuinfo'
> >>module of the VDSM library. Intermediate benefit is that current hooks
> >>and library code can access the information related to host's cpu.
> >>
> >>This allows for moving fake_kvm code into a hook that I've called
> >>fakearch. Fakearch is, in my opinion, more suitable name - there is
> >>barely any KVM faking, but the host 'fakes' selected architecture.
> >>
> >>Faqemu is, on the other hand, a hook. Unfortunately it wasn't really
> >>updated and doesn't allow running VMs under fake architecture. The
> >>series therefore try to refactor it to allow cross-arch VMs to be
> >>started (the VM actually uses host architecture, but from engine's
> >>point of view it's running on the faked arch).
> >
> >so it will run in full emulation, or?
> 
> That is tricky to answer. It is running full emulation in host's
> architecture, hiding the differences between x86_64 and ppc64le by
> modifying the XML e.g. spice->vnc when running x86_64 fakearch on
> ppc64le host.
> 
> The reason for that is the qemu-kvm-rhev doesn't seem to support
> emulation of different arch, so the hook tries to 'fake' it.
> 
> The implication is the fact that the underlying (libvirt) VM is quite
> different to from what we ask for. I wouldn't recommend doing anything
> with the VM apart from running it and killing it.

It is called "fake" for a good reason. It should never be used for
anything but to fake the existence of a VM to the rest of oVirt.

... which leads me to one tiny comment: I like the name faqemu. I don't
see a real need for establishing a new hook with a new name.


> 
> >
> >>
> >>So far tested cross-arch runs are * x86_64->x86_64 (faqemu functionality),
> >>* x86_64->ppc64le (the most important one),
> >>* ppc64le->x86_64,
> >>* ppc64le->ppc64le (faqemu for Power).
> >>
> >>I'm interested in your reviews and comments regarding this effort!
> >>
> >>[1] https://gerrit.ovirt.org/#/c/46962/