Re: Xen 4.18 pvshim console issue (with patch)

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

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.

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