Re: [Xen-devel] Buggy interaction of live migration and p2m updates

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-11-21 at 10:24 +0000, Andrew Cooper wrote:
> On 21/11/14 09:43, Ian Campbell wrote:
> 
> > On Thu, 2014-11-20 at 18:28 +0000, Andrew Cooper wrote:
> > > Realistically, this means no updates to the
> > > p2m at all, due to several potential race conditions.
> > From the rest of the mail it seems as if you are talking primarily about
> > changes to the p2m *structure*, i.e. which guest frames contain the p2m
> > pages, rather than changes to the p2m entries themselves. Is that
> > correct?
> 
> I cover both, although the structure changes are the more obvious, and
> more complicated to fix.

Sure.

> There are race conditions in both existing implementation between a
> p2m/m2p update and the toolstack sampling the dirty bitmap where stale
> frames don't get resent.
> 
> > 
> > I don't see any (explicit) mention of the pfn_to_mfn_frame_list_list
> > here, where does that fit in?
> > 
> 
> It is referenced several times, although not by its exact name.

Hence no explicit mention.

It's ambiguous when you refer to "higher level frames" (which I presume
are the reference you are referring to) because some kernels (perhaps
only historic ones, I've not been keeping up) keep both an N-level tree
of their own internally and the toolstack visible frame_list_list
(sometimes partially overlapping at some level). Is every reference to
"higher level frames" actually intended to be a reference to
pfn_to_mfn_frame_list_list or not?

It seems like sometimes you are talking at times about tracking the
kernel's internal structure and not just pfn_to_mfn_frame_list_list and
I'm not sure why that would be. I'm also not sure why
pfn_to_mfn_frame_list_list is apparently discounted in the linear case,
AFAIK the guest is still obliged to keep that up to date regardless of
the scheme it uses internally for accessing the p2m.

Ian.


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