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virtio-pci: add virtio_pci_optimal_num_queues() helper

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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.

The function handles guests with large numbers of CPUs by limiting the
number of queues to fit within the following constraints:
1. The maximum number of MSI-X vectors.
2. The maximum number of virtqueues.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200818143348.310613-4-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-08-18 15:33:44 +01:00 • committed by Michael S. Tsirkin
parent 3d7e78aa77
commit 1436f32a84
2 changed files with 41 additions and 0 deletions
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9
hw/virtio/virtio-pci.h
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@ -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