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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Oct 02, 2025 at 04:56:48PM +0200, Alejandro Vallejo wrote:
> 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@citrix.com>
> >> >> 
> >> >> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
> >> >> 
> >> >> 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.

The emulated serial is possibly equivalent to the 0xe9 hack in QEMU,
as I expect the serial is mostly setup to "just work".  On native you
likely need to configure the baud rate &c, plus implement flow
control?

Is there any testing XTF should do to ensure the serial is there,
before blindly writing at 0x3f8?

Thanks, Roger.