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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 27, 2014 at 05:19:23PM +0800, Li Liu wrote:
> 
> 
> On 2014/10/26 19:52, Michael S. Tsirkin wrote:
> > On Sat, Oct 25, 2014 at 04:24:52PM +0800, john.liuli wrote:
> >> From: Li Liu <john.liuli@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?
> 
> Yeah, vhost don't need to set ISR under MSI-X mode. But for ARM
> without MSI-X kind mechanism guest can't get the interrupt reason
> through the only one irq hanlder (with one gsi resource).
>
> So I build a shared memory region to provide the interrupt reason
> instead of ISR regiser by qemu without bothering vhost. Then even
> there's only one irq with only one irq hanlder, it still can
> distinguish why irq occur.
> 
> Li.

OK so this might allow sharing IRQs between config and VQs
which might be a handy optimization even for PCI
(at the moment reading ISR causes an exit).

Need to see how all this would work on MP systems though.


> 
> > 
> >> 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
> >>
> > 
> > .
> >