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Subject: Re: [PATCH v1 2/2] virtio-mmio: add features for virtio-mmio specification version 3
On 2019/12/25 äå10:50, Zha Bin wrote:
From: Liu Jiang <gerry@linux.alibaba.com> Userspace VMMs (e.g. Qemu microvm, Firecracker) take advantage of using virtio over mmio devices as a lightweight machine model for modern cloud. The standard virtio over MMIO transport layer only supports one legacy interrupt, which is much heavier than virtio over PCI transport layer using MSI. Legacy interrupt has long work path and causes specific VMExits in following cases, which would considerably slow down the performance: 1) read interrupt status register 2) update interrupt status register 3) write IOAPIC EOI register We proposed to update virtio over MMIO to version 3[1] to add the following new features and enhance the performance. 1) Support Message Signaled Interrupt(MSI), which increases the interrupt performance for virtio multi-queue devices 2) Support per-queue doorbell, so the guest kernel may directly write to the doorbells provided by virtio devices. The following is the network tcp_rr performance testing report, tested with virtio-pci device, vanilla virtio-mmio device and patched virtio-mmio device (run test 3 times for each case): netperf -t TCP_RR -H 192.168.1.36 -l 30 -- -r 32,1024 Virtio-PCI Virtio-MMIO Virtio-MMIO(MSI) trans/s 9536 6939 9500 trans/s 9734 7029 9749 trans/s 9894 7095 9318 [1] https://lkml.org/lkml/2019/12/20/113
Thanks for the patch. Two questions after a quick glance:1) In PCI we choose to support MSI-X instead of MSI for having extra flexibility like alias, independent data and address (e.g for affinity) . Any reason for not start from MSI-X? E.g having MSI-X table and PBA (both of which looks pretty independent). 2) It's better to split notify_multiplexer out of MSI support to ease the reviewers (apply to spec patch as well)
Thanks
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