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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 > >> > >> Reviewed-by: Andrew Cooper > >> > >> 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.