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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.08.2019 17:57, Andrew Cooper wrote:
> On 07/08/2019 16:08, Jan Beulich wrote:
> > 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.
> Right...
>
> I'm confused as to what in my sentence is in any way unclear.
I'm sorry, I must have been completely blind to the "HVM" in
what you've said.

> > > 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).
> ?  I didn't say it was a bug in Xen, but the change is specifically to
> fix a bug.
Whatever we do in Xen, it'll only allow to work around that issue.
An actual fix belongs in the kernel(s). For this reason I suppose
what we're talking about here is a feature (from Xen's pov), not a
bug fix. And it being a feature, it should preferably be coded in
a way that's usable also going forward.

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel