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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/03/15 15:11, Juergen Gross wrote:
> 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.

I concur.  The start info seems like the best place for it.

>
> 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?

The toolstack only needs to access the p2m when doing suspend/resume for
migrate, or a coredump.  None of these are applicable to dom0, so I
think dom0 can get away with doing whatever it prefers in this regard.

~Andrew

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