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

[Rusty Russell <rusty@rustcorp.com.au>]

On Tue, 01 Nov 2011 17:20:06 -0500, Anthony Liguori <aliguori@us.ibm.com> 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.
> 
> Regards,

Can you revert this?  We just reverted the kernel part, figuring we need
a rethink.

Thanks,
Rusty.