On Thu, 26 Oct 2023 at 17:19, Laszlo Ersek <ler...@redhat.com> wrote: > > On 10/26/23 16:21, 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. > > > > (As far as I'm aware usually a boot via EDK2 doesn't pass the > > dtb on to Linux, though I guess there's no reason it can't.) > > > > From QEMU's point of view, we provide two UARTs to the guest, and we > > don't really care whether that means one is used by EDK2 and one by > > Linux, or both are used as getty terminals by Linux, or whether the > > Linux guest uses one serial as a terminal and leaves the other to its > > userspace programs -- it's all just guest software to us :-) > > > > [snip other technical stuff] > > Thanks, good point -- I wasn't aware of the ACPI impact. > > We don't edit / patch QEMU's ACPI tables, ever. (Beyond obeying the ACPI > linker/loader script.) That's a principle we've upheld many times. > Whenever ACPI content needs to change, that implies a QEMU patch. > > So, for this purpose, only the following could have a chance of working: > > - Expose a new config option on the QEMU command line to the user, > regarding the intended use of the serial port(s). This could be of any > tolerable form (machine property, front-end (device) property, whatever > -- anything that QEMU reviewers can accept). > > - In QEMU, generate both the DT and the ACPI tables accordingly. The > ACPI tables would have to immediately *not* contain the UART-to-hide (so > as to keep it secret from the guest OS). The DT at the same time would > still have to expose the "runtime DEBUG UART", because edk2 would have > to know where that UART was (and that it was meant specifically for OS > runtime debug output). > > - Edk2 would have to patch the DT (we tend to do that already), because > (in some configs) we do forward the DT to the guest OS. This need for > patching could be lifted if QEMU adopted such a form of expression for > the "runtime DEBUG UART" that would be ignored by Linux out of the box. > > > > >> All in all, I think the implementation would be quite a steep divergence > >> from, or on top of, this patch set. :) > > > > I agree with this and with Ard's "not something to address in this > > series" comment above; it doesn't sound like this is something that > > needs to hold up the patchset we have currently. > > Right; I'd like to flush this one. The runtime debug UART seems to need > more joint pondering. > > > > > Does anybody have time to review Laszlo's code? It would be nice > > to be able to get this into the next EDK2 release. >
I'm happy for this to go in if it covers our needs. Acked-by: Ard Biesheuvel <a...@kernel.org> -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#110115): https://edk2.groups.io/g/devel/message/110115 Mute This Topic: https://groups.io/mt/101834880/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
