Re: [Qemu-ppc] [Qemu-devel] [PATCH 3/4] ppc: add CPU access_type into the migration stream

[David Gibson]

On Wed, Sep 13, 2017 at 06:17:21PM +0100, Mark Cave-Ayland wrote:
> On 13/09/17 08:19, David Gibson wrote:
> 
> >> When pausing a VM, does execution stop at the end of the current TB
> >> rather than immediately? If so, perhaps someone could confirm that
> >> guarantee is good enough for access_type?
> > 
> > I'm pretty sure it has to; we'd have to come up out of an individual
> > TB in order to get to the main loop where we check the "please pause"
> > flag.  I'm not sure if that helps us here though - I *think* access
> > type is about carrying information from the point where we trigger an
> > exception to the point where we actually start processing the
> > exception.
> > 
> > This code is really ugly and I've never understood it well :(. It's
> > always seemed bogus to me that we have an essentially global variable
> > to carry information over that small gap, though.  Unfortunately it's
> > unlikely that I'd be able to dive into this and work out if it's
> > really needed any time soon.
> 
> >From my testing yesterday it does appear to be required for TCG (or
> unless this is exposing another bug in the Mac migration) so I can check
> it works here and then maybe someone else confirm it works on KVM?
> 
> A couple of things I've noticed: the heathrow VMStateDescription looks
> good, however I can see that the OpenPIC timers won't likely migrate
> correctly without adding a few more fields - that's easy to fix.

Right.  And since OpenPIC isn't used on any platforms that have real
production use in the wild, it's fine to bump the migration stream
version for it.

> Another thing is that if we're changing the migration stream for Mac
> models Ben has some OpenPIC and other related changes in his wip queue
> that it might make sense to put those in first before properly fixing
> the Mac machine migration.

That would have something to be said for it.

It's probably not essential, though, since I don't consider the
non-pseries platforms at this stage sufficiently mature that we
guarantee a stable migration stream format.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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