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Re: [PATCH for-4.21] x86/hvm: fix reading from 0xe9 IO port if port E9 hack is active
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 2 Oct 2025 15:38:05 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
- Delivery-date: Thu, 02 Oct 2025 13:38:34 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
> CC-ing Oleksii as you've tagged this for 4.21.
I was told that bugfixes didn't need a release-ack until hard code
freeze, which is the 31st of October?
Thanks, Roger.
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