Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.08.2019 17:00, Andrew Cooper wrote:
> On 07/08/2019 15:30, Jan Beulich wrote:
> > On 07.08.2019 15:41, Andrew Cooper wrote:
> > > Furthermore, if there is this problem for event channels, then there is
> > > almost certainly a related problem for grant tables.
> > >
> > > The control in Xen should be expressed in a positive form, or the logic
> > > will become a tangle.  It should be a bit permitting the use of the FIFO
> > > ABI, rather than a bit saying "oh actually, you can't use that".
> > >
> > > That said, it might be easier to declare FIFO to be "event channel v2",
> > > and specify max_{grant,evntchn}_ver instead.
> > I'm not sure assuming linear (or actually any) ordering between
> > variants is a good thing. Yes, right now we only have gnttab
> > v1 and v2 and evtchn 2l and fifo, which could be considered v1
> > and v2 as you suggest. However, assuming a 3rd variant surfaces,
> > why would it be that one has to expose v2 just to make v3
> > usable? In particular gnttab v2 has various issues (which is why
> > you introduced a way to disable its use in the first place), yet
> > I'd hope we'd end up with a less quirky v3 if one ever becomes
> > necessary. And in turn I'd hope we could hide v2 from any v3
> > users.
> >
> > IOW I think a bitmap to permit use of "advanced" versions is
> > more future proof. (As a side note, I don't think we want to
> > introduce a disable for the respective v1 interfaces.)
> We absolutely do want a way to turn everything off.
>
> The inability to turn the Xen extensions off for HVM guests (even if
> only for debugging purposes) is a severely short sighted decision.
For HVM perhaps, but not for PV.

> It is also a feature which has been requested repeatedly by users in the
> past, and I am very deliberately building a way to do this into the
> CPUID work.
>
> However, it is an unreasonable request to bundle into this bugfix, hence
> why I didn't suggest it.
There's no bug fix here, as there's no bug (in Xen).

> Now I think about it, things like available ABIs really should be in the
> Xen hypervisor CPUID leaves, but again, that ship sailed a decade ago.
For comparison, do you know of any CPU architecture making _all_
of its basic insns and other functionality available conditionally
only? Just like there, I think it is reasonable to have a basic
set available in all cases. Nevertheless, as said above, HVM might
have benefited from making even some basic hypercalls conditional,
because there they're strictly extensions, not base functionality.

Jan

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