Re: [PATCH v3 1/8] serial: fake IRQ-regs context in poll handlers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2024 15:53, Marek Marczykowski wrote:
> On Tue, Feb 13, 2024 at 08:45:54AM +0100, Jan Beulich wrote:
>> On 13.02.2024 04:43, Marek Marczykowski wrote:
>>> On Mon, Feb 12, 2024 at 10:04:38AM +0100, Jan Beulich wrote:
>>>> On 08.02.2024 23:00, Julien Grall wrote:
>>>>> On 05/02/2024 13:27, Jan Beulich wrote:
>>>>>> In preparation of dropping the register parameters from
>>>>>> serial_[rt]x_interrupt() and in turn from IRQ handler functions,
>>>>>> register state needs making available another way for the few key
>>>>>> handlers which need it. Fake IRQ-like state.
>>>>>>
>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>>> ---
>>>>>> The use of guest_cpu_user_regs() in dbc_uart_poll() is inconsistent with
>>>>>> other console poll functions we have, and it's unclear whether that's
>>>>>> actually generally correct.
>>>>>
>>>>> Is it? Looking at ns16550_poll() we would pass guest_user_regs() if 
>>>>> run_in_exception() doesn't exist. But looking at the caller, no-on seems 
>>>>> to care about the 'regs'. So is this just a latent bug?
>>>>
>>>> What do you mean by "doesn't exist"? ns16550_poll() assumes it exists.
>>>> And I can spot any use of guest_user_regs() on the respective generic
>>>> or Arm-specific bug.c paths.
>>>>
>>>>> BTW, do you have an idea why the poll function is not run in an 
>>>>> exception handler?
>>>>
>>>> "The poll function" being which one? If you mean the one in xhci-dbc.c
>>>> then that's why I had Cc-ed Marek. Moving him to To: - maybe that
>>>> manages to finally catch his attention.
>>>
>>> TBH, I don't know. That's part of the original xue patch at
>>> https://github.com/connojd/xue/blob/master/patches/xen-xue-dbgp.patch
>>> and it works for me as it is.
>>
>> "Works" meaning what? Doesn't crash on you? Or does also provide
>> sensible output in _all_ cases (i.e. including when e.g. the poll
>> happens to run on an idle vCPU)?
> 
> Generally provides sensible output, for example during boot (it is using
> idle vCPU then, right?).

Before Dom0 is started: Yes. With the exception of the phase where PV
Dom0's page tables are constructed, albeit in that time window
guest_cpu_user_regs() shouldn't yield sensible data either. I can only
say I'm surprised; since I have no way to properly test with an XHCI
debug port, I'd have to see about faking something to convince myself
(unless you were to supply example output).

Jan