Re: [PATCH] tools: Remove support for qemu-trad's battery reporting

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

On Wed Mar 26, 2025 at 1:59 PM GMT, Marek Marczykowski-Górecki wrote:
> On Tue, Mar 25, 2025 at 05:41:10PM +0000, Alejandro Vallejo wrote:
> > The way this undocumented feature works is via qemu-trad (who nobody
> > uses anymore), by intercepting 3 magic PIOs. 0x88 is one of them, and
> > it's probed by hvmloader as a means of detecting support for this (so,
> > on qemu-upstream this check always fails). If hvmloader detects the
> > feature, it appends an SSDT with AML inherited from some laptop ~20y
> > ago. QEMU then communicates with a userspace daemon (xenpmd) via an
> > undocumented xenstore key ("refreshbatterystatus") in order to report
> > battery levels.
> > 
> > Seeing how no one uses, mantains or cares about qemu-trad anymore, rip
> > it all out. The hvmloader check, the SSDT generation logic and xenpmd.
>
> Oh, I didn't know something like this existed!

In retrospect, it might've been for the best. I really dislike the way it's put
together. Using xenstore feels really pointless.

> We needed a feature like this, and solved it via extra kernel module +
> PV-like interface to feed it with data from dom0:
> https://github.com/QubesOS/qubes-dummy-psu/

I did wonder (after learning how this all works) how you guys did it without
qemu-trad. I guess that explains it. FWIW, it's not hard to do this properly on
QEMU upstream. We could create a new field under a BAR of the Xen platform
device and instruct some (much, much, much simpler) AML to read the battery
level from there. Then QEMU can ask the real system what the battery level is
and Bob's your uncle.

But...

>
> I guess it doesn't make much sense for me to resurrect the old interface
> and bring it to QEMU upstream, as I need the battery info in PVH too.
> So, I'm fine with removing it as is.

... in principle I'd say QubesOS and OpenXT are the two only plausible users of
this feature. So it's a tad pointless if you've since developed a working
solution many years ago.

Cheers,
Alejandro