Re: [Qemu-devel] [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and older compat types

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Aug 29, 2012 at 01:21:13AM +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 28, 2012 at 07:02:42PM -0300, Eduardo Habkost wrote:
> > On Wed, Aug 29, 2012 at 12:35:28AM +0300, Michael S. Tsirkin wrote:
> > > On Tue, Aug 28, 2012 at 04:13:38PM -0300, Eduardo Habkost wrote:
> > > > On Tue, Aug 28, 2012 at 08:43:52PM +0300, Michael S. Tsirkin wrote:
> > > > > In preparation for adding PV EOI support, disable PV EOI by default for
> > > > > 1.1 and older machine types, to avoid CPUID changing during migration.
> > > > > 
> > > > > PV EOI can still be enabled/disabled by specifying it explicitly.
> > > > >     Enable for 1.1
> > > > >     -M pc-1.1 -cpu kvm64,+kvm_pv_eoi
> > > > >     Disable for 1.2
> > > > >     -M pc-1.2 -cpu kvm64,-kvm_pv_eoi
> > > > > 
> > > > 
> > > > What about users that are already running "qemu-1.1 -M pc-1.1" on a host
> > > > kernel that supports PV EOI already? They would get PV EOI disabled when
> > > > migrating to a destination running "qemu-1.2 -M pc-1.1".
> > > > 
> > > > (On the other hand, people running "qemu-1.1 -M pc-1.1" on a host kernel
> > > > supporting PV EOI already have migration broken, so there's not much we
> > > > can do for them)
> > > 
> > > Exactly.
> > > 
> > > Talked to Gleb, long term I think we should rework code to make
> > > it forward-compatible wrt adding new MSRs:
> > > - source gets list of MSRs to be migrated from KVM and simply sends them all
> > > - send all MSRS in key/value format
> > > - destination gets list of MSRs to be migrated from KVM and
> > >   only restores the supported ones
> > 
> > As far as I understand the migration code requirements/expectations, if
> > the origin is sending some data, it is because it is part of the
> > guest-visible machine state that must be kept while migrating. Because
> > of that, the destination is not allowed to drop anything it doesn't know
> > about.
> 
> 
> We have a ton of code that reads in values then just
> ignores them, for compat with old qemu.
> This will be exactly such a case:
> we don't drop anything - protocol does not
> support this. We read and simply do not tell kvm
> about it. We also have tons of code that sends
> useless values again for compatibility.
> 
> > At the same time, if it's not part of guest-visible machine
> > state, it doesn't have to be sent by the migration origin.
> > 
> 
> False, we often send internal device state which is not
> directly guest visible.
> 
> > On the other hand, a mode of operation that doesn't require updating
> > QEMU every time there's a new bit of guest-visible state to be migrated
> > would be nice (just like the "-cpu host" mode, that doesn't require
> > updating QEMU for every new CPU feature, is nice for some use cases). I
> > just don't know how to make work with the current migration protocol.
> > 
> 
> I don't understand. What is the problem with the proposal?
> What will not work with our protocol?
> Can you give an example please?
> 
> 
> > > Too late for 1.2?
> > 
> > Absolutely (in my opinion).

My concern here is that you are introducing infrastructure by using
_one example_, claiming it to solve a general problem, without the
appropriate amount of energy spent in this solution.

Using old machines is supposed to be supported on older guests.

Michael, not migrating the PVEOI MSR actually crashes the guest?