Re: [Qemu-devel] [PATCH 6/8] i386/kvm: hv-stimer requires hv-time and hv-synic

[Vitaly Kuznetsov]

Divya Garg <divya.garg@nutanix.com> writes:

> Hi Vitaly Kuznetsov !
> I was working on hyperv flags and saw that we introduced new 
> dependencies some
> time back 
> (https://sourcegraph.com/github.com/qemu/qemu/-/commit/c686193072a47032d83cb4e131dc49ae30f9e5d7?visible=1).
> After these changes, if we try to live migrate a vm from older qemu to newer
> one having these changes, it fails showing dependency issue.
>
> I was wondering if this is the expected behaviour or if there is any work
> around for handing it ? Or something needs to be done to ensure backward
> compatibility ?

Hi Divya,

configurations with 'hv-stimer' and without 'hv-synic'/'hv-time' were
always incorrect as Windows can't use the feature, that's why the
dependencies were added. It is true that it doesn't seem to be possible
to forward-migrate such VMs to newer QEMU versions. We could've tied
these new dependencies to newer machine types I guess (so old machine
types would not fail to start) but we didn't do that back in 4.1 and
it's been awhile since... Not sure whether it would make much sense to
introduce something for pre-4.1 machine types now.

Out of curiosity, why do such "incorrect" configurations exist? Can you
just update them to include missing flags on older QEMU so they migrate
to newer ones without issues?

-- 
Vitaly