When tag_set->nr_maps is 1, the block layer limits the number of hw queues by nr_cpu_ids. No matter how many hw queues are use by virtio-blk/virtio-scsi, as they both have (tag_set->nr_maps == 1), they can use at most nr_cpu_ids hw queues. In addition, specifically for pci scenario, when the 'num-queues' specified by qemu is more than maxcpus, virtio-blk/virtio-scsi would not be able to allocate more than maxcpus vectors in order to have a vector for each queue. As a result, they fall back into MSI-X with one vector for config and one shared for queues. Considering above reasons, this patch set limits the number of hw queues used by nr_cpu_ids for both virtio-blk and virtio-scsi. ------------------------------------------------------------- Here is test result of virtio-scsi: qemu cmdline: -smp 2,maxcpus=4, \ -device virtio-scsi-pci,id=scsi0,num_queues=8, \ -device scsi-hd,drive=drive0,bus=scsi0.0,channel=0,scsi-id=0,lun=0, \ -drive file=test.img,if=none,id=drive0 Although maxcpus=4 and num_queues=8, 4 queues are used while 2 interrupts are allocated. # cat /proc/interrupts ... ... 24: 0 0 PCI-MSI 65536-edge virtio0-config 25: 0 369 PCI-MSI 65537-edge virtio0-virtqueues ... ... # /sys/block/sda/mq/ 0 1 2 3 ------> 4 queues although qemu sets num_queues=8 With the patch set, there is per-queue interrupt. # cat /proc/interrupts 24: 0 0 PCI-MSI 65536-edge virtio0-config 25: 0 0 PCI-MSI 65537-edge virtio0-control 26: 0 0 PCI-MSI 65538-edge virtio0-event 27: 296 0 PCI-MSI 65539-edge virtio0-request 28: 0 139 PCI-MSI 65540-edge virtio0-request 29: 0 0 PCI-MSI 65541-edge virtio0-request 30: 0 0 PCI-MSI 65542-edge virtio0-request # ls /sys/block/sda/mq 0 1 2 3 ------------------------------------------------------------- Here is test result of virtio-blk: qemu cmdline: -smp 2,maxcpus=4, -device virtio-blk-pci,drive=drive-virtio-disk0,id=virtio-disk0,num-queues=8 -drive test.img,format=raw,if=none,id=drive-virtio-disk0 Although maxcpus=4 and num-queues=8, 4 queues are used while 2 interrupts are allocated. # cat /proc/interrupts ... ... 24: 0 0 PCI-MSI 65536-edge virtio0-config 25: 0 65 PCI-MSI 65537-edge virtio0-virtqueues ... ... # ls /sys/block/vda/mq 0 1 2 3 -------> 4 queues although qemu sets num_queues=8 With the patch set, there is per-queue interrupt. # cat /proc/interrupts 24: 0 0 PCI-MSI 65536-edge virtio0-config 25: 64 0 PCI-MSI 65537-edge virtio0-req.0 26: 0 10290 PCI-MSI 65538-edge virtio0-req.1 27: 0 0 PCI-MSI 65539-edge virtio0-req.2 28: 0 0 PCI-MSI 65540-edge virtio0-req.3 # ls /sys/block/vda/mq/ 0 1 2 3 Reference: https://lore.kernel.org/lkml/e4afe4c5-0262-4500-aeec-60f30734b4fc@default/ Thank you very much! Dongli Zhang
