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

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

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.

---
 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;
-- 
1.7.9.111.gf3fb0