Re: [Xen-devel] EFI development issues

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.01.2020 21:28, Andrew Cooper wrote:
> On 13/01/2020 16:46, Jan Beulich wrote:
>> On 13.01.2020 17:02, Andrew Cooper wrote:
>>>   (Waiting for a
>>> keypress on StdIn however does work, which is how I eventually diagnosed
>>> that it was an output problem.)  Skipping this logic allows debugging to
>>> work.
>> As should then do -basevideo.
> 
> That is another observation.  It is very confusing having command line
> parameters to xen.efi which are different to command line parameters to
> Xen.  The parameters to xen.efi aren't documented anywhere, as far as I
> can tell.

Well, if "xen.efi -help" doesn't count, then yes, this may want adding
to efi.pandoc. The -cfg option is mentioned there (albeit not in a
systematic manner), and the only other possibly useful option that was
there from the beginning was -basevideo; -mapbs appeared only later.
When creating that doc, focus was clearly on the commonly used aspects.

>>> What is the purpose of changing to a different mode?  Certainly as far
>>> as serial consoles go, sticking with the mode the loader uses certainly
>>> feels like a safer option.
>> Does a serial console report a "resolution" in the first place?
> 
> How would I find out?

You could log the values found in case of a pure serial console
setup (i.e. headless). But the multiplexed case is going to be the
more involved one anyway.

>> And
>> if we were able to (sufficiently easily) tell video from serial
>> console, how would we deal with the case of StdOut / StdErr being
>> multiplexed to both?
> 
> In my case they are multiplexed, and all work fine before set_mode() call.
> 
> After set_mode(), VGA still works, while serial sees nothing further on
> StdOut/Stderr (pre-existing lines are still in place).

Unfortunately the two of the three EFI boxes I routinely work with
and that I've tried mux-ed configurations on had no such issue, so
I'm afraid I couldn't easily debug this myself even if I wanted to.
But anyway, situations like this one are exactly why -basevideo has
been there from the very beginning.

For systems with a video device, the mode set here may be the mode
an eventual X driver also uses; sadly Linux'es kernel mode setting
looks to be having a high (and increasing) failure rate according
to my experience. Possibly much like the high rate of EFI issues
you observe / get reported, as opposed to me not having seen most
of them on the boxes I work with all the time (i.e. simply bad vs
good luck).

Jan

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