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Re: [Qemu-devel] [RFC PATCH 0/2] virtio-mmio: add irqfd support for vhos
Michael S. Tsirkin
Re: [Qemu-devel] [RFC PATCH 0/2] virtio-mmio: add irqfd support for vhost-net based on virtio-mmio
Mon, 27 Oct 2014 12:48:45 +0200
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 <address@hidden>
> >> 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.
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(-)
> >> --
> >> 22.214.171.124
> > .