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

[Liu, Jing2]

On 2/11/2020 11:17 AM, Jason Wang wrote:
> 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?
Actually this bit *aims* for msi sharing or msi non-sharing.

It means, when msi sharing bit is 1, device doesn't want vector per queue

(it wants msi vector sharing as name) and doesn't want a high interrupt 
rate.

So driver turns to !per_vq_vectors and has to do dynamical mapping.

So they are opposite not superset.

Thanks!

Jing


> Thanks
>
>
>
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