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

[Anthony Liguori <aliguori@us.ibm.com>]

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,

Anthony Liguori

> ---
>   hw/virtio-pci.c |    2 ++
>   1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
> index df27c19..68cd5bc 100644
> --- a/hw/virtio-pci.c
> +++ b/hw/virtio-pci.c
> @@ -659,6 +659,8 @@ void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev)
>                             "virtio-pci", size);
>       pci_register_bar(&proxy->pci_dev, 0, PCI_BASE_ADDRESS_SPACE_IO,
>                        &proxy->bar);
> +    pci_register_bar(&proxy->pci_dev, 2, PCI_BASE_ADDRESS_SPACE_MEMORY,
> +&proxy->bar);
>
>       if (!kvm_has_many_ioeventfds()) {
>           proxy->flags&= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;