Re: Xen 4.18 pvshim console issue (with patch)

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Wed, Oct 18, 2023 at 03:51:54PM +0200, Jan Beulich wrote:
> On 18.10.2023 15:36, Manuel Bouyer wrote:
> > On Wed, Oct 18, 2023 at 03:24:22PM +0200, Jan Beulich wrote:
> >> On 18.10.2023 13:20, Manuel Bouyer wrote:
> >>> On Wed, Oct 18, 2023 at 11:44:22AM +0100, Andrew Cooper wrote:
> >>>> On 18/10/2023 11:38 am, Manuel Bouyer wrote:
> >>>>> Hello,
> >>>>> With Xen 4.18, a PV domain running under pvshim doesn't get console 
> >>>>> input.
> >>>>> This is because the domain id in pvshim isn't 0 (and on x86 
> >>>>> max_init_domid is
> >>>>> hardwired to 0), so console_input_domain() will never select that domain
> >>>>> as input.
> >>>>>
> >>>>> The attached patch fixes it by translating 0 to the real domain id for
> >>>>> pvshim, but there may be a better way to do this.
> >>>>>
> >>>>
> >>>> Thankyou for the report.
> >>>>
> >>>> First, CC'ing Henry as 4.18 release manager.
> >>>>
> >>>> There have been changes in how this works recently in 4.18, notably c/s
> >>>> c2581c58bec96.
> >>>
> >>> Yes, it looks like this one introduced the problem.
> >>> Before this, we would switch console_rx to 1 without checking if
> >>> domain (console_rx - 1) exists, and console_rx == 1 is a special case
> >>> in __serial_rx()
> >>>
> >>>>
> >>>> However, it's not obvious whether this worked in 4.17 or not.  i.e.
> >>>> whether it's a regression in 4.18, or whether it's been broken since PV
> >>>> Shim was introduced.
> >>>
> >>> I don't know for 4.16 or 4.17 but I can tell that it's working in 4.15
> >>
> >> >From looking at the code, it doesn't look like it would: There
> >> switch_serial_input() toggles console_rx between 0 and 1 afaict, and
> >> console_input_domain() handles value 0 as "Xen" (like in 4.18), while
> >> otherwise it calls rcu_lock_domain_by_id(console_rx - 1) (i.e. trying
> >> to get hold of Dom0's domain structure, not Dom1's).
> > 
> > Well, I have a 4.15.5 installation in production and I can tell you that
> > with PV+PVSHIM, the console is working, for sure.
> > 
> > AFAIK console_input_domain() is called only on ARM, from
> > vpl011_write_data_xen(). It's never called for x86.
> 
> Oh, indeed. I took your patch touching the function as meaning it's used
> on x86. This then means my earlier suggestion won't work, as we need
> console_rx to have the value 1 for input to be accepted from Dom1. Which
> in turn means your change to switch_serial_input(), suitably cleaned up,
> is then likely the best we can do, despite me not really liking the shim
> special casing.
> 
> As to cleaning up - besides the build, indentation, and style issues I
> think you want to drop the "&& pv_shim" part of the condition (as
> get_initial_domain_id() takes care of that already), and ideally you'd
> also move the new "domid" variable into the more narrow scope. If you
> don't feel like providing a proper (updated) patch, I'm happy to take
> over, but to retain indication of your initial work I'd need you to
> permit me to add your S-o-b (alongside mine).

No problem, please do !

-- 
Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
     NetBSD: 26 ans d'experience feront toujours la difference
--