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

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

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.