Re: [edk2-devel] [PATCH 0/9] ArmVirtPkg: support two PL011 UARTs

["Ard Biesheuvel" <ardb@kernel.org>]

On Thu, 26 Oct 2023 at 16:46, Julien Grall <julien@xen.org> wrote:
>
> Hi,
>
> On 26/10/2023 15:21, Peter Maydell wrote:
> > On Tue, 10 Oct 2023 at 16:33, Laszlo Ersek <lersek@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.
>

I'd much rather have an implementation of the _STA method on those
devices where the underlying AML queries the fwcfg MMIO device


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