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Re: [Xen-devel] [PATCH] x86: add feature flags to shared_info page



On 03/02/2015 02:56 PM, Jan Beulich wrote:
On 02.03.15 at 14:44, <andrew.cooper3@xxxxxxxxxx> wrote:
On 02/03/15 13:15, Jan Beulich wrote:
On 02.03.15 at 13:59, <"jgross@xxxxxxxx".non-mime.internet> wrote:
In order to indicate the Xen tools capability to support the virtual
mapped linear p2m list instead the 3 level mfn tree add feature flags
to the shared_info page.
But why in the shared info page? They'd belong to start info or should
be obtainable via XENVER_get_features.

Furthermore in this case, the virtual linear p2m is purely a
guest->toolstack feature/ABI.

Xen deliberately has no knowledge of PV guest p2ms of either the 3level
or linear variety.

Is there genuinely no better interface than the hypervisor feature flags
to indicate a piece of toolstack support?

As Ian indicated in his reply, much depends on whether any other
mechanism would allow the information to be retrieved early enough
in the guest.

Options that would work are, regarding the time the information is
needed:

- XENVER_get_features
- start info
- shared info
- any (other) hypercall

All other interfaces are available much too late.

I think start info is the best option, as it is built by the tools for
domUs and, as Jan already mentioned, would be a better place for the
information as shared info.

The last remaining question: what to do regarding dom0? Here I see the
following alternatives:

- do nothing: the 3 level mfn tree is built even if it is not needed,
  wastes up to 2MB memory and might slow down dom0 (I doubt the slow
  down would be detectable)
- set the flag based on a hypervisor boot parameter (not very nice)
- throw the 3 level mfn tree away during boot as soon as the tools can
  tell dom0 to do so (requires a new interface)

Any thoughts?


Juergen

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