[RFC PATCH 0/2] virtio-mmio: add irqfd support for vhost-net based on virtio-mmio

Michael S. Tsirkin
Sun, 26 Oct 2014 11:52:10 -0000

On Sat, Oct 25, 2014 at 04:24:52PM +0800, john.liuli
wrote:
> From: Li Liu <john.liuli at huawei.com>
>
> This set of patches try to implemet irqfd support of vhost-net
> based on virtio-mmio.
>
> I had posted a mail to talking about the status of vhost-net
> on kvm-arm refer to http://www.spinics.net/lists/kvm-arm/msg10804.html.
> Some dependent patches are listed in the mail too. Basically the
> vhost-net brings great performance improvements, almost 50%+.
>
> It's easy to implement irqfd support with PCI MSI-X. But till
> now arm32 do not provide equivalent mechanism to let a device
> allocate multiple interrupts. And even the aarch64 provid LPI
> but also not available in a short time.
>
> As Gauguey Remy said "Vhost does not emulate a complete virtio
> adapter but only manage virtqueue operations". Vhost module
> don't update the ISR register, so if with only one irq then it's
> no way to get the interrupt reason even we can inject the
> irq correctly.

Well guests don't read ISR in MSI-X mode so why does it help
to set the ISR bit?

> To get the interrupt reason to support such VIRTIO_NET_F_STATUS
> features I add a new register offset VIRTIO_MMIO_ISRMEM which
> will help to establish a shared memory region between qemu and
> virtio-mmio device. Then the interrupt reason can be accessed by
> guest driver through this region. At the same time, the virtio-mmio
> dirver check this region to see irqfd is supported or not during
> the irq handler registration, and different handler will be assigned.
>
> I want to know it's the right direction? Does it comply with the
> virtio-mmio spec.? Or anyone have more good ideas to emulate mis-x
> based on virtio-mmio? I hope to get feedback and guidance.
> Thx for any help.
>
> Li Liu (2):
>   Add a new register offset let interrupt reason available
>   Assign a new irq handler while irqfd enabled
>
>  drivers/virtio/virtio_mmio.c |   55
+++++++++++++++++++++++++++++++++++++++---
>  include/linux/virtio_mmio.h  |    3 +++
>  2 files changed, 55 insertions(+), 3 deletions(-)
>
> --
> 1.7.9.5
>