Re: [Xen-devel] [PATCH V2] Add flag to start info regarding virtual mapped p2m list

[Juergen Gross <jgross@xxxxxxxx>]

On 03/18/2015 10:59 AM, Ian Campbell wrote:
> On Tue, 2015-03-17 at 06:50 +0100, Juergen Gross wrote:
> > On 03/04/2015 12:41 PM, Ian Campbell wrote:
> > > On Wed, 2015-03-04 at 12:22 +0100, Juergen Gross wrote:
> > > > It would either be used like intended,
> > > Which is how? That is what is really missing here.
> > >
> > > So far this appears to be a bit which enables some as yet unspecified[0]
> > > behaviour in one particular OS kernel with some as yet undiscussed
> > > potential future impact on toolstack functionality.
> > Sorry for the late answer, but I managed to move this mail to my archive
> > before reacting on it. :-(
> No problem.
>
> The description looks like the sort of thing which ought to go into the
> commit log.
Okay, I can change it.

> Is there not an ABI change somewhere else relating to the exposure of
> the cr3+vaddr+size? If so why is it not in this patch?
Commit 50bd1f0825339dfacde471df7664729216fc46e3

> Ideally whichever file which needs to change in xen/include/public to
> expose that change should also come along with documentation for this
> new ABI.
Included in above commit in form of comments in the modified file.

> If that change has been deferred for some reason then I think it (and
> why) should be mentioned in the commit message, you'll also want to
> explain why adding the bit now but the ABI change later is safe, i.e.
> what the transition plan is.
>
> AIUI this change has broken memory hotplug and has also made it
> difficult from an ABI PoV to reinstate that support. I think that needs
> to be addressed (i.e. at the ABI design level, not necessary
> implemented) before we add a bit exposing this feature.
The interface change didn't brake anything. It was the implementation in
the Linux kernel.

> > Guests implementing this new interface need to know, of course, whether
> > the Xen tools are capable to use the new interface instead of the old
> > p2m tree interface. Otherwise a guest using only the new interface with
> > the virtual mapped linear p2m list on a machine with old Xen tools not
> > supporting this interface could not be restored or migrated.
> What is the mechanism for the converse, i.e. for the tools to detect if
> a given guest is making use of this functionality?
See above commit. It's all in the comments.

> Are tools going to be able to tell at migration time, or is it something
> which needs to be detectable at domain build time (and therefore exposed
> in the kernel ELF notes)?
Migration time is fine.


Juergen

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