[Qemu-devel] Re: [PATCH 0/6] Save state error handling (kill off no_migrate)

[Michael S. Tsirkin]

On Mon, Nov 08, 2010 at 07:59:57AM -0700, Alex Williamson wrote:
> On Mon, 2010-11-08 at 13:40 +0200, Michael S. Tsirkin wrote:
> > On Wed, Oct 06, 2010 at 02:58:57PM -0600, Alex Williamson wrote:
> > > Our code paths for saving or migrating a VM are full of functions that
> > > return void, leaving no opportunity for a device to cancel a migration,
> > > either from error or incompatibility.  The ivshmem driver attempted to
> > > solve this with a no_migrate flag on the save state entry.  I think the
> > > more generic and flexible way to solve this is to allow driver save
> > > functions to fail.  This series implements that and converts ivshmem
> > > to uses a set_params function to NAK migration much earlier in the
> > > processes.  This touches a lot of files, but bulk of those changes are
> > > simply s/void/int/ and tacking a "return 0" to the end of functions.
> > > Thanks,
> > > 
> > > Alex
> > 
> > Well error handling is always tricky: it seems easier to
> > require save handlers to never fail.
> 
> Sure it's easier, but does that make it robust?

More robust in the face of wwhat kind of failure?

> > So there's a bunch of code here but what exactly is the benefit?
> > Since save handlers have no idea what does the remote do,
> > what is the compatibility you mention?
> 
> There are two users I currently have in mind.  ivshmem currently makes
> use of the register_device_unmigratable() because it makes use of host
> specific resources and connections (aiui).  This sets the no_migrate
> flag, which is not dynamic and a bit of a band-aide.
>  The other is
> device assignment, which needs a way to NAK a migration since physical
> devices are never migratable.

Well since all these can't be migrated ever, a fixed property actually seems
a good match.  Sure it's not dynamic but all the easier to debug.

>  I imagine we could at some point have
> devices with state tied to other features that can't always be detached
> from the host, this tries to provide the infrastructure for that to
> happen.
> 
> Alex

Let guest control whether you can migrate?
Sounds like something that is more likely to be abused
than used constructively. 

-- 
MST