@@ -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
@@ -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,
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(+)