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

[Claudio Fontana <claudio.fontana@huawei.com>]

On 13.01.2015 06:46, David Gibson wrote:
> 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 <david@gibson.dropbear.id.au>
> 

Can you guys take a look also at this:

http://lists.nongnu.org/archive/html/qemu-devel/2015-01/msg01048.html

I really think we should give diagnostics under PCI_DEBUG when trying to set new BAR addresses fails for any reason.

Ciao,

Claudio