[libvirt] [PATCH] docs: formatdomain: explain host-model/host-passthrough requirements

[Dr. David Alan Gilbert]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> Unlike host-model, host-passthrough does not do any check, it just passes
> "-cpu host" blindly. Therefore, lack of flags can and will result in
> crashes---SIGSEGV if it happens in userspace, but worse if for example the
> destination doesn't have an MSR that should be there according to the
> kernel.

Ah OK, yes.

Dave

> Paolo
> 
> Il mar 6 ago 2019, 18:22 Dr. David Alan Gilbert <dgilbert at redhat.com> ha
> scritto:
> 
> > * Paolo Bonzini (pbonzini at redhat.com) wrote:
> > > host-passthrough documentation menions that the source and destination
> > > hosts are not identical in both hardware and configuration.
> > Configuration
> > > actually includes microcode version and QEMU version, but this is not
> > > clear so make it explicit
> > >
> > > Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> > > ---
> > >  docs/formatdomain.html.in | 12 ++++++------
> > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
> > > index f5c882141a..6d084d7c04 100644
> > > --- a/docs/formatdomain.html.in
> > > +++ b/docs/formatdomain.html.in
> > > @@ -1590,8 +1590,8 @@
> > >            hand, the ABI provided to the guest is reproducible. During
> > >            migration, complete CPU model definition is transferred to the
> > >            destination host so the migrated guest will see exactly the
> > same CPU
> > > -          model even if the destination host contains more capable CPUs
> > for
> > > -          the running instance of the guest; but shutting down and
> > restarting
> > > +          model for the running instance of the guest, even if the
> > destination
> > > +          host contains more capable CPUs or newer kernel; but shutting
> > down and restarting
> > >            the guest may present different hardware to the guest
> > according to
> > >            the capabilities of the new host. Prior to libvirt 3.2.0 and
> > QEMU
> > >            2.9.0 detection of the host CPU model via QEMU is not
> > supported.
> > > @@ -1625,10 +1625,10 @@
> > >            environment cannot be reproduced on different hardware. Thus,
> > if you
> > >            hit any bugs, you are on your own. Further details of that
> > CPU can
> > >            be changed using <code>feature</code> elements. Migration of
> > a guest
> > > -          using host-passthrough is dangerous if the source and
> > destination
> > > -          hosts are not identical in both hardware and configuration.
> > If such
> > > -          a migration is attempted then the guest may hang or crash upon
> > > -          resuming execution on the destination host.</dd>
> > > +          using host-passthrough is dangerous if the source and
> > destination hosts
> > > +          are not identical in both hardware, QEMU version, microcode
> > version
> > > +          and configuration. If such a migration is attempted then the
> > guest may
> > > +          hang or crash upon resuming execution on the destination
> > host.</dd>
> >
> > If libvirt is  driving qemu with 'exact' on the cpu option, under what
> > situation would it hang/crash on the destination?
> > All of the microcode changes I think have caused a flag to appear, and
> > thus it would be required on the destination.
> >
> > Dave
> >
> > >          </dl>
> > >
> > >          Both <code>host-model</code> and <code>host-passthrough</code>
> > modes
> > > --
> > > 2.21.0
> > >
> > --
> > Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> >
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK