Re: Xen 4.18 pvshim console issue (with patch)

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan