Re: [PATCH v1 2/2] virtio-mmio: add features for virtio-mmio specification version 3

[Jason Wang <jasowang@redhat.com>]

On 2019/12/25 上午10:50, 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