[SeaBIOS] [Qemu-devel] [PATCH v4 for-2.3 13/25] hw/acpi: remove from root bus 0 the crs resources used by other busses.
Kevin O'Connor
kevin at koconnor.net
Sun Mar 8 19:26:10 CET 2015
On Sun, Mar 08, 2015 at 07:51:42PM +0200, Marcel Apfelbaum wrote:
> On 03/08/2015 06:13 PM, Kevin O'Connor wrote:
> >If I read this correctly, it looks like a machine with two root buses
> >and 20 devices, each with one memory range and one io range, would end
> >up with 40 CRS ranges (ie, a CRS range for every resource).
> Correct.
>
> As Michael pointed out in another thread, the firmware is considered
> guest code and QEMU cannot assume anything on how the resources are
> assigned. This is why this solution was chosen.
>
> However we have two things that make the situation a little better.
> 1. The PXB implementation includes a pci-bridge and all devices are automatically
> attached to the secondary bus, in this way we have one IO/MEM range per extra root bus.
Out of curiosity, does the PXB implementation add the pci-bridge just
to simplify the IO/MEM range, or are there other technical reasons for
it?
> 2. On top of this series we can add a merge algorithm that will bring together
> consecutive ranges. This series does not include this optimization and it
> focuses on the correctness.
>
> It also
> >looks like this furthers the requirement that the guest firmware
> >assign the PCI resources prior to QEMU being able to generate the ACPI
> >tables.
> >
> >Am I correct? If so, that doesn't sound ideal.
> You are correct, however is not that bad because we have the following sequence:
> - Early in the boot sequence the bios scans the PCI buses and assigns IO/MEM ranges
> - At this moment all resources needed by QEMU are present in the configuration space.
> - At the end of the boot sequence the BIOS queries the ACPI tables and *only then*
> the tables are computed.
>
> I think we use that implicitly for other features, anyway, it looks like an elegant
> solution with no real drawbacks. (Our assumptions are safe)
Thank you for the clarification. I understand that it works, but I've
never been that comfortable with the QEMU<->firmware dance with PCI
resources. I do understand that the alternatives have as many or more
problems though. So, I'm not objecting to this implementation.
Cheers,
-Kevin
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