Re: [PATCH v2 00/35] Introduce NS8250 UART emulator

[Denis Mukhin <dmkhn@xxxxxxxxx>]

On Saturday, December 14th, 2024 at 10:05 AM, Marek Marczykowski-Górecki 
<marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:

>
>
> On Thu, Dec 05, 2024 at 08:41:30PM -0800, Denis Mukhin via B4 Relay wrote:
>
> > The patch series introduces initial in-hypervisor emulator for
> > NS8250/NS16x50-compatible UARTs under CONFIG_HAS_VUART_NS8250.
> >
> > In parallel domain creation scenario (hyperlaunch), NS8520 emulator helps
> > early guest OS bringup debugging, because it eliminates dependency on the
> > external emulator being operational by the time domains are created. Also,
> > there's no early console facility similar to vpl011 to support x86 guest OS
> > bring up.
> >
> > The NS8250 emulator is disabled by default.
>
>
> On a high level, why the mechanism used by earlyprintk=xen (IIUC i/o
> port 0xe9) isn't enough?

I/O port 0xe9 needs some knowledge about Xen and potentially 
re-building/patching
guest OSes. The latter is not always possible right away. So using legacy
COM ports makes debugging a bit easier for engineers trying to bring up
Xen configuration.

> Hyperlaunch can't start full (Xen-unaware) HVM domains anyway, so
> a requirement to use a Xen-specific interface for the console shouldn't be
> an issue, no?

With hyperlaunch virtual firmware (e.g. OVMF) in HVM case will execute
in parallel, which means all serial output from virtual firmware can be
easily captured, i.e. debugging on a new hardware should be a bit easier.

My understanding that not all firmware support debug I/O port 0xe9.
Rebuilding a firmware may be a quest in itself, which can be skipped if x86
port of Xen has legacy COM UART emulation in hypervisor.

Also, my understanding (please correct me if I'm wrong) it should be
theoretically possible to craft a hyperlaunch configuration where each
non-hardware domain is headless and has its own physical block device
controller passed-through and has no dependency on hardware domain.

>
> --
> Best Regards,
> Marek Marczykowski-Górecki
> Invisible Things Lab