Re: [PATCH for-4.21] x86/hvm: fix reading from 0xe9 IO port if port E9 hack is active

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Thu Oct 2, 2025 at 4:17 PM CEST, Roger Pau Monné wrote:
> On Thu, Oct 02, 2025 at 04:02:00PM +0200, Alejandro Vallejo wrote:
>> On Thu Oct 2, 2025 at 3:38 PM CEST, Roger Pau Monné wrote:
>> > On Thu, Oct 02, 2025 at 11:37:36AM +0100, Andrew Cooper wrote:
>> >> On 02/10/2025 11:22 am, Roger Pau Monne wrote:
>> >> > Reading from the E9 port if the emergency console is active should 
>> >> > return
>> >> > 0xe9 according to the documentation from Bochs:
>> >> >
>> >> > https://bochs.sourceforge.io/doc/docbook/user/bochsrc.html
>> >> >
>> >> > See `port_e9_hack` section description.
>> >> >
>> >> > Fix Xen so it also returns the port address.  OSes can use it to detect
>> >> > whether the emergency console is available or not.
>> >> >
>> >> > Fixes: d1bd157fbc9b ("Big merge the HVM full-virtualisation 
>> >> > abstractions.")
>> >> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> >> 
>> >> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> >> 
>> >> That's been wrong for rather a long time.  How did you find it?
>> >
>> > I came across the documentation above and I didn't remember Xen
>> > returning any value for reads, which sadly was indeed true.
>> >
>> > This was because I had the intention to suggest Alejandro to (also?) use
>> > the port 0xe9 hack for printing from XTF, which should work for both
>> > Xen and QEMU.
>> 
>> QEMU doesn't support 0xE9 though?
>
> AFAICT it does:
>
> https://wiki.osdev.org/QEMU#The_debug_console
>
> However when running QEMU on Xen as a device model writes to 0xe9 are
> handled in the hypervisor, and never forwarded to any device model.
>
> Regards, Roger.

The more you know. I searched for it once upon a time and couldn't find it.
Thanks for the pointer.

Regardless, "-debugcon stdio" is functionally equivalent to "-serial stdio"
and the latter can be adapted to work on real hardware too.

Cheers,
Alejandro