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Re: [PATCH for-4.21] x86/hvm: fix reading from 0xe9 IO port if port E9 hack is active
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- Date: Thu, 2 Oct 2025 16:02:00 +0200
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- Cc: <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 02 Oct 2025 14:02:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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?
Cheers,
Alejandro
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