On 10/26/23 17:21, Ard Biesheuvel wrote: > 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>
Thank you, Ard! Merged as commit range 74c687cc2f2f..f945b72331d7 via <https://github.com/tianocore/edk2/pull/4967>. Laszlo -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#110141): https://edk2.groups.io/g/devel/message/110141 Mute This Topic: https://groups.io/mt/101834880/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/leave/9847357/21656/1706620634/xyzzy [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
