Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support

[Jason Wang]

On 2020/2/10 äå5:05, Zha Bin wrote:
> From: Liu Jiang<gerry@xxxxxxxxxxxxxxxxx>
>
> 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 add MSI support for virtio over MMIO via new feature
> bit VIRTIO_F_MMIO_MSI[1] which increases the interrupt performance.
>
> With the VIRTIO_F_MMIO_MSI feature bit supported, the virtio-mmio MSI
> uses msi_sharing[1] to indicate the event and vector mapping.
> Bit 1 is 0: device uses non-sharing and fixed vector per event mapping.
> Bit 1 is 1: device uses sharing mode and dynamic mapping.


I believe dynamic mapping should cover the case of fixed vector?

Thanks