[PATCH v4 1/5] virtio-pci: add virtio_pci_optimal_num_queues() helper

[Stefan Hajnoczi <stefanha@redhat.com>]

Multi-queue devices achieve the best performance when each vCPU has a
dedicated queue. This ensures that virtqueue used notifications are
handled on the same vCPU that submitted virtqueue buffers.  When another
vCPU handles the the notification an IPI will be necessary to wake the
submission vCPU and this incurs a performance overhead.

Provide a helper function that virtio-pci devices will use in later
patches to automatically select the optimal number of queues.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/virtio/virtio-pci.h | 9 +++++++++
 hw/virtio/virtio-pci.c | 7 +++++++
 2 files changed, 16 insertions(+)

diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
index e2eaaa9182..91096f0291 100644
--- a/hw/virtio/virtio-pci.h
+++ b/hw/virtio/virtio-pci.h
@@ -243,4 +243,13 @@ typedef struct VirtioPCIDeviceTypeInfo {
 /* Register virtio-pci type(s).  @t must be static. */
 void virtio_pci_types_register(const VirtioPCIDeviceTypeInfo *t);
 
+/**
+ * virtio_pci_optimal_num_queues:
+ * @fixed_queues: number of queues that are always present
+ *
+ * Returns: The optimal number of queues for a multi-queue device, excluding
+ * @fixed_queues.
+ */
+unsigned virtio_pci_optimal_num_queues(unsigned fixed_queues);
+
 #endif
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index d028c17c24..0c4f0100ca 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -19,6 +19,7 @@
 
 #include "exec/memop.h"
 #include "standard-headers/linux/virtio_pci.h"
+#include "hw/boards.h"
 #include "hw/virtio/virtio.h"
 #include "migration/qemu-file-types.h"
 #include "hw/pci/pci.h"
@@ -2024,6 +2025,12 @@ void virtio_pci_types_register(const VirtioPCIDeviceTypeInfo *t)
     g_free(base_name);
 }
 
+unsigned virtio_pci_optimal_num_queues(unsigned fixed_queues)
+{
+    /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */
+    return MIN(current_machine->smp.cpus, VIRTIO_QUEUE_MAX - fixed_queues);
+}
+
 /* virtio-pci-bus */
 
 static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
-- 
2.25.4