virtio-pci: reduce modern_mem_bar size

Currently each VQ Notification Virtio Capability is allocated
on a different page. The idea is to enable split drivers within
guests, however there are no known plans to do that.
The allocation will result in a 8MB BAR, more than various
guest firmwares pre-allocates for PCI Bridges hotplug process.

Reserve 4 bytes per VQ by default and add a new parameter
"page-per-vq" to be used with split drivers.
Signed-off-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -294,6 +294,12 @@ static void virtio_pci_ioeventfd_set_disabled(DeviceState *d, bool disabled)
 
 #define QEMU_VIRTIO_PCI_QUEUE_MEM_MULT 0x1000
 
+static inline int virtio_pci_queue_mem_mult(struct VirtIOPCIProxy *proxy)
+{
+    return (proxy->flags & VIRTIO_PCI_FLAG_PAGE_PER_VQ) ?
+        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT : 4;
+}
+
 static int virtio_pci_ioeventfd_assign(DeviceState *d, EventNotifier *notifier,
                                        int n, bool assign)
 {
@@ -307,7 +313,7 @@ static int virtio_pci_ioeventfd_assign(DeviceState *d, EventNotifier *notifier,
     MemoryRegion *modern_mr = &proxy->notify.mr;
     MemoryRegion *modern_notify_mr = &proxy->notify_pio.mr;
     MemoryRegion *legacy_mr = &proxy->bar;
-    hwaddr modern_addr = QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
+    hwaddr modern_addr = virtio_pci_queue_mem_mult(proxy) *
                          virtio_get_queue_index(vq);
     hwaddr legacy_addr = VIRTIO_PCI_QUEUE_NOTIFY;
 
@@ -1370,7 +1376,8 @@ static void virtio_pci_notify_write(void *opaque, hwaddr addr,
                                     uint64_t val, unsigned size)
 {
     VirtIODevice *vdev = opaque;
-    unsigned queue = addr / QEMU_VIRTIO_PCI_QUEUE_MEM_MULT;
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(DEVICE(vdev)->parent_bus->parent);
+    unsigned queue = addr / virtio_pci_queue_mem_mult(proxy);
 
     if (queue < VIRTIO_QUEUE_MAX) {
         virtio_queue_notify(vdev, queue);
@@ -1609,7 +1616,7 @@ static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
         struct virtio_pci_notify_cap notify = {
             .cap.cap_len = sizeof notify,
             .notify_off_multiplier =
-                cpu_to_le32(QEMU_VIRTIO_PCI_QUEUE_MEM_MULT),
+                cpu_to_le32(virtio_pci_queue_mem_mult(proxy)),
         };
         struct virtio_pci_cfg_cap cfg = {
             .cap.cap_len = sizeof cfg,
@@ -1744,8 +1751,7 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
     proxy->device.type = VIRTIO_PCI_CAP_DEVICE_CFG;
 
     proxy->notify.offset = 0x3000;
-    proxy->notify.size =
-        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT * VIRTIO_QUEUE_MAX;
+    proxy->notify.size = virtio_pci_queue_mem_mult(proxy) * VIRTIO_QUEUE_MAX;
     proxy->notify.type = VIRTIO_PCI_CAP_NOTIFY_CFG;
 
     proxy->notify_pio.offset = 0x0;
@@ -1754,8 +1760,8 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
 
     /* subclasses can enforce modern, so do this unconditionally */
     memory_region_init(&proxy->modern_bar, OBJECT(proxy), "virtio-pci",
-                       2 * QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
-                       VIRTIO_QUEUE_MAX);
+                       /* PCI BAR regions must be powers of 2 */
+                       pow2ceil(proxy->notify.offset + proxy->notify.size));
 
     memory_region_init_alias(&proxy->modern_cfg,
                              OBJECT(proxy),
@@ -1833,6 +1839,8 @@ static Property virtio_pci_properties[] = {
                     VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT, false),
     DEFINE_PROP_BIT("x-disable-pcie", VirtIOPCIProxy, flags,
                     VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false),
+    DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags,
+                    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 

hw/virtio/virtio-pci.h

@@ -64,6 +64,7 @@ enum {
     VIRTIO_PCI_FLAG_MIGRATE_EXTRA_BIT,
     VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT,
     VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT,
+    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT,
 };
 
 /* Need to activate work-arounds for buggy guests at vmstate load. */
@@ -84,6 +85,10 @@ enum {
 #define VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY \
     (1 << VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT)
 
+/* page per vq flag to be used by split drivers within guests */
+#define VIRTIO_PCI_FLAG_PAGE_PER_VQ \
+    (1 << VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT)
+
 typedef struct {
     MSIMessage msg;
     int virq;

include/hw/compat.h

@@ -2,7 +2,11 @@
 #define HW_COMPAT_H
 
 #define HW_COMPAT_2_7 \
-        /* empty */
+    {\
+        .driver   = "virtio-pci",\
+        .property = "page-per-vq",\
+        .value    = "on",\
+    },
 
 #define HW_COMPAT_2_6 \
     {\