[ovirt-devel] rethinking fake-kvm and faqemu

[Martin Polednik]

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 :)
>>
>> 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.

>
>>
>> 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/
>>
>> Best regards,
>> mpolednik
>