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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-11-21 at 11:07 +0000, Andrew Cooper wrote:
> On 21/11/14 10:46, Ian Campbell wrote:
> > On Fri, 2014-11-21 at 10:24 +0000, Andrew Cooper wrote:
> >> On 21/11/14 09:43, Ian Campbell wrote:
> >>> 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?
> 
> "higher level frames" would be the toolstack-abi-defined first and
> second level lists.  The logdirty infrastructure can be used to detect
> writes to these frames, and therefore detect structural changes to the p2m.
> 
> I would like to hope that every kernel out there keeps this information
> correctly up-to-date and updates it in an appropriate order...
> 
> >
> > 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 apologise for giving this impression.  It was not intended.

Great, I just wanted to be sure we were all on the same page, since
scrobbling around in the kernel's internal data structures would clearly
be mad...

> 
> > 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.
> 
> There are two reasons for the virtual linear p2m, the primary one being
> to break the hard 512GB limit given the old 3-level table.
> 
> A 64bit PV guest cannot possibly use the pfn_to_mfn_frame_list_list if
> it needs to actually exceed 512GB of RAM.  Therefore, to signal the use
> the virtual linear method, a PV guest explicitly sets
> pfn_to_mfn_frame_list_list to INVALID_MFN, and fills in the brand new
> adjacent information.

Oh, I hadn't realised this linear p2m stuff involved a guest ABI change.
Have I somehow completely missed the xen.git side of these patches? I
thought I'd only seen linux.git ones (and hence wasn't looking very
closely). 

Ian.


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