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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Sat, Dec 14, 2024 at 07:05:15PM +0100, Marek Marczykowski-Górecki 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?
> 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?

I assume the point is to provide a minimal set of non-Xen specific
interfaces so that a guest *could* work if made to use only those
interfaces.  The 0xE9 hack is quite common for emergency printing, but
it doesn't allow for input, which the ns8250 does.

Thanks, Roger.