On Wed, Nov 02, 2011 at 01:52:55PM +1030, Rusty Russell wrote: > On Tue, 01 Nov 2011 17:20:06 -0500, Anthony Liguori <aligu...@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<da...@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.
By the way, it looks like linux guests already do pci_iomap to get at the BAR. And that is supposed to work fine with both memory and IO. So a very simple solution, without touching linux guests, would be to allow changing BAR 0 type to memory in QEMU. It'll break non-linux guests unfortunately, but maybe that's not an issue for PowerVM? -- MST
