Hi,
On 26/10/2023 15:21, Peter Maydell wrote:
On Tue, 10 Oct 2023 at 16:33, Laszlo Ersek <ler...@redhat.com> wrote:
On 10/10/23 09:43, Ard Biesheuvel wrote:
Thanks for looking into this - a cleanup was overdue here.
I will take a look in more detail later, but one thing that occurred
to me when reading this overview is that having a separate DEBUG
serial port would permit us to
a) remove it from the DT
... as in, hide it from Linux, I assume?
b) add a runtime mapping for it
c) keep using it after ExitBootServices
This could be useful for debugging issues with the variable store etc.
Not saying this is something to address in this series, but I'd like
to hear your take on this.
Sounds like a useful feature.
I see four challenges:
(1) We'd have to coordinate it with Peter. If we hide any one of the
serial ports from Linux, that may not be what QEMU intends for Linux to
happen. Linux currently ties getties to all serial ports -- via the
serial* aliases, IIUC. Thus, some "positive identification" in the DT
could be necessary (i.e., that edk2 was welcome to hide that port from
Linux).
The potential awkwardness here is that what the guest thinks about
the serial ports depends on the ACPI table fragments which QEMU
provides. EDK2 would need to edit the table fragment to remove any
mention of the second UART if it wanted to hide it from the kernel.
I don't know how hard that would be in EDK2.
I am not sure if it would help EDK2 in this case. But we had a similar
problem when adding support for ACPI in Xen. It was not trivial to
remove the UART from the ACPI tables provided by the host. So we ended
up to introduce the STAO table [1]. This is used to describe which
device will be hidden to the OS.
Cheers,
[1] https://wiki.xenproject.org/images/0/02/Status-override-table.pdf
--
Julien Grall
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