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

[David Gibson]

On Mon, Jan 12, 2015 at 07:24:06AM -0600, Michael Roth wrote:
> 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.
> 
> Ping

Sorry, I just got back from vacation.

It looks same to me.

Reviewed-by: David Gibson <address@hidden>

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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