Re: [Qemu-devel] [PATCH RFC] virtio-pci: add MMIO property

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 19, 2012 at 04:07:45PM -0500, Anthony Liguori wrote:
> On 03/19/2012 03:49 PM, Michael S. Tsirkin wrote:
> >On Mon, Mar 19, 2012 at 02:19:33PM -0500, Anthony Liguori wrote:
> >>On 03/19/2012 10:56 AM, Michael S. Tsirkin wrote:
> >>>Currently virtio-pci is specified so that configuration of the device is
> >>>done through a PCI IO space (via BAR 0 of the virtual PCI device).
> >>>However, Linux guests happen to use ioread/iowrite/iomap primitives
> >>>for access, and these work uniformly across memory/io BARs.
> >>>
> >>>While PCI IO accesses are faster than MMIO on x86 kvm,
> >>>MMIO might be helpful on other systems which don't
> >>>implement PIO or where PIO is slower than MMIO.
> >>>
> >>>Add a property to make it possible to tweak the BAR type.
> >>>
> >>>Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> >>>
> >>>This is harmless by default but causes segfaults in memory.c
> >>>when enabled. Thus an RFC until I figure out what's wrong.
> >>
> >>Doesn't this violate the virtio-pci spec?
> >>
> >
> >The point is to change the BAR type depending on the architecture.
> >IO is fastest on x86 but maybe not on other architectures.
> 
> Are we going to document that the BAR is X on architecture Y in the spec?
> 
> I think the better way to do this is to use a separate device id
> range for MMIO virtio-pci.  You can make the same driver hand both
> ranges and that way the device is presented consistently to the
> guest regardless of what the architecture is.

Yes there are endless ways to do this.
This specific hack is good for making existing linux drivers
on ppc, arm etc work.

> >>Making the same vendor/device ID have different semantics depending
> >>on a magic flag in QEMU seems like a pretty bad idea to me.
> >>
> >>Regards,
> >>
> >>Anthony Liguori
> >
> >We do this with MSI-X so why not the BAR type?
> 
> We extend the bar size with MSI-X and use a transport flag to
> indicate that it's available, right?

No, we use regular pci capability. Just like BAR type is
a regular PCI register :)

> Regards,
> 
> Anthony LIguori
> 
> >
> >>>
> >>>---
> >>>  hw/virtio-pci.c |   16 ++++++++++++++--
> >>>  hw/virtio-pci.h |    4 ++++
> >>>  2 files changed, 18 insertions(+), 2 deletions(-)
> >>>
> >>>diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
> >>>index 28498ec..6f338d2 100644
> >>>--- a/hw/virtio-pci.c
> >>>+++ b/hw/virtio-pci.c
> >>>@@ -655,6 +655,7 @@ void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev)
> >>>  {
> >>>      uint8_t *config;
> >>>      uint32_t size;
> >>>+    uint8_t bar0_type;
> >>>
> >>>      proxy->vdev = vdev;
> >>>
> >>>@@ -684,8 +685,14 @@ void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev)
> >>>
> >>>      memory_region_init_io(&proxy->bar,&virtio_pci_config_ops, proxy,
> >>>                            "virtio-pci", size);
> >>>-    pci_register_bar(&proxy->pci_dev, 0, PCI_BASE_ADDRESS_SPACE_IO,
> >>>-&proxy->bar);
> >>>+
> >>>+    if (proxy->flags&   VIRTIO_PCI_FLAG_USE_MMIO) {
> >>>+        bar0_type = PCI_BASE_ADDRESS_SPACE_MEMORY;
> >>>+    } else {
> >>>+        bar0_type = PCI_BASE_ADDRESS_SPACE_IO;
> >>>+    }
> >>>+
> >>>+    pci_register_bar(&proxy->pci_dev, 0, bar0_type,&proxy->bar);
> >>>
> >>>      if (!kvm_has_many_ioeventfds()) {
> >>>          proxy->flags&= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
> >>>@@ -823,6 +830,7 @@ static Property virtio_blk_properties[] = {
> >>>      DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true),
> >>>      DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 2),
> >>>      DEFINE_VIRTIO_BLK_FEATURES(VirtIOPCIProxy, host_features),
> >>>+    DEFINE_PROP_BIT("mmio", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_MMIO_BIT, false),
> >>>      DEFINE_PROP_END_OF_LIST(),
> >>>  };
> >>>
> >>>@@ -856,6 +864,7 @@ static Property virtio_net_properties[] = {
> >>>      DEFINE_PROP_UINT32("x-txtimer", VirtIOPCIProxy, net.txtimer, TX_TIMER_INTERVAL),
> >>>      DEFINE_PROP_INT32("x-txburst", VirtIOPCIProxy, net.txburst, TX_BURST),
> >>>      DEFINE_PROP_STRING("tx", VirtIOPCIProxy, net.tx),
> >>>+    DEFINE_PROP_BIT("mmio", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_MMIO_BIT, false),
> >>>      DEFINE_PROP_END_OF_LIST(),
> >>>  };
> >>>
> >>>@@ -888,6 +897,7 @@ static Property virtio_serial_properties[] = {
> >>>      DEFINE_PROP_HEX32("class", VirtIOPCIProxy, class_code, 0),
> >>>      DEFINE_VIRTIO_COMMON_FEATURES(VirtIOPCIProxy, host_features),
> >>>      DEFINE_PROP_UINT32("max_ports", VirtIOPCIProxy, serial.max_virtserial_ports, 31),
> >>>+    DEFINE_PROP_BIT("mmio", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_MMIO_BIT, false),
> >>>      DEFINE_PROP_END_OF_LIST(),
> >>>  };
> >>>
> >>>@@ -915,6 +925,7 @@ static TypeInfo virtio_serial_info = {
> >>>
> >>>  static Property virtio_balloon_properties[] = {
> >>>      DEFINE_VIRTIO_COMMON_FEATURES(VirtIOPCIProxy, host_features),
> >>>+    DEFINE_PROP_BIT("mmio", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_MMIO_BIT, false),
> >>>      DEFINE_PROP_END_OF_LIST(),
> >>>  };
> >>>
> >>>@@ -969,6 +980,7 @@ static int virtio_scsi_exit_pci(PCIDevice *pci_dev)
> >>>  static Property virtio_scsi_properties[] = {
> >>>      DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 2),
> >>>      DEFINE_VIRTIO_SCSI_PROPERTIES(VirtIOPCIProxy, host_features, scsi),
> >>>+    DEFINE_PROP_BIT("mmio", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_MMIO_BIT, false),
> >>>      DEFINE_PROP_END_OF_LIST(),
> >>>  };
> >>>
> >>>diff --git a/hw/virtio-pci.h b/hw/virtio-pci.h
> >>>index e560428..e6a8861 100644
> >>>--- a/hw/virtio-pci.h
> >>>+++ b/hw/virtio-pci.h
> >>>@@ -24,6 +24,10 @@
> >>>  #define VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT 1
> >>>  #define VIRTIO_PCI_FLAG_USE_IOEVENTFD   (1<<   VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT)
> >>>
> >>>+/* Some guests don't support port IO. Use MMIO instead. */
> >>>+#define VIRTIO_PCI_FLAG_USE_MMIO_BIT 2
> >>>+#define VIRTIO_PCI_FLAG_USE_MMIO   (1<<   VIRTIO_PCI_FLAG_USE_MMIO_BIT)
> >>>+
> >>>  typedef struct {
> >>>      PCIDevice pci_dev;
> >>>      VirtIODevice *vdev;
> >