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

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

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.

> 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?

> >
> >---
> >  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;