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Re: [Qemu-devel] [PATCH v4 for-2.3 13/25] hw/acpi: remove from root bus

From: Kevin O'Connor
Subject: Re: [Qemu-devel] [PATCH v4 for-2.3 13/25] hw/acpi: remove from root bus 0 the crs resources used by other busses.
Date: Sun, 8 Mar 2015 14:26:10 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

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

> 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.


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