Re: [Xen-devel] [PATCH 15/22] vixen: pass through version hypercalls to parent Xen

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/01/2018 15:40, Anthony Liguori wrote:
> On Sun, Jan 7, 2018 at 12:31 AM, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>> On Sat, Jan 06, 2018 at 02:54:30PM -0800, Anthony Liguori wrote:
>>> From: Anthony Liguori <aliguori@xxxxxxxxxx>
>>>
>>> This is necessary to trigger event channel upcalls but it is also
>> I'm lost here, what does version have to do with upcalls?
> In Linux, xen_force_evtchn_callback() does HYPERVISOR_xen_version(0,
> NULL).  This is done when IRQs are re-enabled after being disabled to
> trigger checking pending.
>
> I'm not 100% confident that it's necessary to pass this all the way
> through to the parent Xen but it seemed like the right thing to do
> since we need the parent to update pending events in order for the
> events in Vixen to get updated.
>
>>> useful to passthrough the full version information such that the
>>> guest believes it is running on the parent Xen.
>> In any case, I think this is wrong. The interface the guest sees is
>> the interface from vixen, not the interface of the L0. Hence reporting
>> the L0 version is not appropriate.
> I think it depends on what you want.  We were aiming for maximum
> compatibility and many users trigger behavior from Xen version for
> better or worse.
>
> Happy to make this optional if this isn't universally desired.

It will be subtle either way.

My gut feeling is that it will be worse to pretend that Xen 4.10 isn't
4.10, than having PV guests suddenly find themselves on a newer
hypervisor.  The PV ABI hasn't changed much at all.

I don't have any evident to back up this feeling though.

~Andrew

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