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

[Juergen Gross <jgross@suse.com>]

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. :-(

Xen pv domains are using a domain private p2m list to convert guest pfns
to mfns. This p2m list has to be updated by the Xen tools during domain
restore and migration, as the mfns will most likely change. In order to
locate the p2m list the Xen tools need an interface provided by the
guest. Up to now this interface has been the shared info page where the
guest would store the mfn of the top level page of a 3-level p2m tree.

This p2m tree is fixed in it's layout and due to the limitation of
entries it can hold at each level it is limiting the maximum size of the
p2m list which can be reported to the Xen tools. The maximum memory the
p2m tree can support for 64 bit domains is 512 GB (32 bit domains don't
have a problem, as the p2m tree limit is much higher than the supported
domain size of 64 GB).

In order to be able to support pv domains with more than 512 GB an
additional way to specify the p2m list for the Xen tools has been added:
instead of a tree structure linked via mfns, the virtual address of a
linear p2m list, the cr3 value of the related address space and the size
of the p2m list can be specified by the guest.

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.

The added flag in the start info indicates the Xen tool's capability to
use the new interface enabling the guest to omit the p2m tree and thus
to support more than 512 GB of RAM.


Juergen