Re: [Qemu-devel] [PATCH 0/1] pci: allow 0 address for PCI IO/MEM regions

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Michael Roth (2014-12-23 13:33:35)
> This patch enables the programming of address 0 for IO/MMIO BARs for
> PCI devices.
> 
> It was originally included as part of a series implementing PCI
> hotplug for pseries guests, where it is needed due to the fact
> that pseries guests access IO space via MMIO, and that IO
> space is dedicated to PCI devices, with RTAS calls being used in
> place of common/legacy IO ports such as config-data/config-address.
> 
> Thus, the entire range is unhindered by legacy IO ports, and
> pseries guest kernels may attempt to program an IO BAR to address 0
> as a result.
> 
> This has led to a conflict with the existing PCI config space
> emulation code, where it has been assumed that 0 address are always
> invalid.
> 
> Some background from discussions can be viewed here:
> 
>   https://lists.nongnu.org/archive/html/qemu-devel/2014-08/msg03063.html
> 
> The general summary from that discussion seems to be that 0-addresses are
> not (at least, are no longer) prohibited by current versions of the PCI
> spec, and that the same should apply for MMIO addresses (where allowing
> 0-addresses are also needed for some ARM-based PCI controllers).
> 
> This patch includes support for 0-address MMIO BARs based on that
> discussion.
> 
> One still-lingering concern is whether this change will impact
> compatibility with guests where 0-addresses are invalid. There was
> some discussion on whether this issue could be addressed using
> memory region priorities, but I think that's still an open question
> that we can hopefully address here.

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