Re: [Qemu-devel] [PATCH] virtio: Add PCI memory BAR in addition to PIO BAR

[Anthony Liguori <anthony@codemonkey.ws>]

On 11/01/2011 07:16 PM, David Gibson wrote:
> On Tue, Nov 01, 2011 at 05:20:06PM -0500, Anthony Liguori wrote:
>> On 09/30/2011 12:26 AM, David Gibson wrote:
>>> Currently, virtio devices are usually presented to the guest as an
>>> emulated PCI device, virtio_pci.  Although the actual IO operations
>>> are done through system memory, the configuration of the virtio device
>>> is done through the one PCI IO space BAR that virtio_pci presents.
>>>
>>> But PCI IO space (aka PIO) is deprecated for modern PCI devices, and
>>> on some systems with many PCI domains accessing PIO space can be
>>> problematic.  For example on the existing PowerVM implementation of
>>> the PAPR spec, PCI PIO access is not supported at all.  We're hoping
>>> that our KVM implementation will support PCI PIO (once we support PCI
>>> at all), but it will probably have some irritating limitations.
>>>
>>> This patch, therefore, extends the virtio_pci device to have a PCI
>>> memory space (MMIO) BAR as well as the IO BAR.  The MMIO BAR contains
>>> exactly the same registers, in exactly the same layout as the existing
>>> PIO BAR.
>>>
>>> Because the PIO BAR is still present, existing guest drivers should
>>> still work fine.  With this change in place, future guest drivers can
>>> check for an MMIO BAR and use that if present (falling back to PIO
>>> when possible to support older qemu versions).
>>>
>>> Signed-off-by: David Gibson<david@gibson.dropbear.id.au>
>>
>> Seems harmless for QEMU, so applied.  You should update the
>> virtio-pci spec too.
>
> Ugh, sorry.  This is the old version of the patch which breaks when
> you actually use it because it attempts to give proxy->bar two
> different parents.  I have a new version which does it correctly.
>
> So, do you want to revert and reapply, or should I just make a fix
> patch?

I'll revert.  Thanks.

Regards,

Anthony Liguori