diff options
| -rw-r--r-- | hw/virtio/virtio-pci.c | 32 | ||||
| -rw-r--r-- | hw/virtio/virtio-pci.h | 9 |
2 files changed, 41 insertions, 0 deletions
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c index ccdf54e81c..fc69570dcc 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" @@ -2058,6 +2059,37 @@ 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 the same vCPU that submitted + * virtqueue buffers can handle their completion. When a different vCPU + * handles completion it may need to IPI the vCPU that submitted the + * request and this adds overhead. + * + * Virtqueues consume guest RAM and MSI-X vectors. This is wasteful in + * guests with very many vCPUs and a device that is only used by a few + * vCPUs. Unfortunately optimizing that case requires manual pinning inside + * the guest, so those users might as well manually set the number of + * queues. There is no upper limit that can be applied automatically and + * doing so arbitrarily would result in a sudden performance drop once the + * threshold number of vCPUs is exceeded. + */ + unsigned num_queues = current_machine->smp.cpus; + + /* + * The maximum number of MSI-X vectors is PCI_MSIX_FLAGS_QSIZE + 1, but the + * config change interrupt and the fixed virtqueues must be taken into + * account too. + */ + num_queues = MIN(num_queues, PCI_MSIX_FLAGS_QSIZE - fixed_queues); + + /* + * There is a limit to how many virtqueues a device can have. + */ + return MIN(num_queues, VIRTIO_QUEUE_MAX - fixed_queues); +} + /* virtio-pci-bus */ static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size, 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 |