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

[Juergen Gross <jgross@suse.com>]

On 03/02/2015 02:56 PM, Jan Beulich wrote:
>>>> On 02.03.15 at 14:44, <andrew.cooper3@citrix.com> wrote:
>> On 02/03/15 13:15, Jan Beulich wrote:
>>>>>> On 02.03.15 at 13:59, <"jgross@suse.com".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