Re: Migration of a 32-bit PV-Guest

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 21/01/15 15:17, Vasco Fachin wrote:
> Hi everybody,
>
> I am currently working on a object-oriented paravirtual operating
> system as a university project.

Sounds like an interesting project.

>
> We were able to use a MiniOS core in order to build a relative stable
> guest, or at least stable enough for our purposes. :-D
>
> However for one thing we could not proceed. We need to migrate our
> guest to another machine, so after some research we came by this
> hypercall: HYPERVISOR_update_va_mapping(...), which is apparently
> needed in order to unmap the shared info page before we could suspend
> our system.

Yes - the shared info page must be removed from the pagetables before
suspend, and must be remapped after resume.  The shared info page is
actually a Xen page mapped into the guest, so will be in a different
location.

>
> At this point, our system crashes everytime with the following output
> from "xm dmesg":
> (XEN) mm.c:918:d320 Error getting mfn 4636 (pfn 5555555555555555) from
> L1 entry 0000000004636023 for l1e_owner=320, pg_owner=320

pfn 5555555555555555 looks like a deliberately poisoned value, which
suggests that you are not umapping the page that you are intending to.

Can you observe what the shared info mfn is from boot?  I presume it
isn't 0x4636 given the above error.

>
> I tested the same hypercall with a 32bit version of MiniOS and it
> crashes as well with the same output. Then I tried it with a 64bit
> version of MiniOS and seems to work just fine.
>
> Somehow I cannot find any 32bit paravirtual machine that could be
> migrated. Does anybody know if such a system exists?

Any 32bit Linux guest will successfully migrate.

>
> Our configuration at the moment is this:
> Dom0: Ubuntu 12.10 64bit
> XEN: xen 4.1.4 64bit
> DomU: 32bit PV-Guest (Object-oriented derived from a 32bit version of MiniOS).
>
> We tested our system, with the same behavior, also on the following platform:
> Dom0: Ubuntu 12.04 32bit
> XEN: xen 4.1.4 32bit
>
> My question is: does anybody know if a migration of a 32bit PV-System
> is possible (in Xen 4.1.4/any newer release)? Or has anyone an idea of
> why this particular hypercall fails to execute on a 32bit guest, while
> it works on a 64bit guest?

I can tell you for absolute certain that 32bit PV guests can migrate. 
We test this extensively in XenServer, and I was quite careful to make
sure it continued to work even after rewriting the migration toolstack
code for migration v2.

~Andrew