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

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Date:	Sun, 26 Oct 2014 13:52:10 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"john.liuli" <john.liuli@...wei.com>
Cc:	linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
	peter.huangpeng@...wei.com, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org,
	n.nikolaev@...tualopensystems.com, yingshiuan.pan@...il.com,
	remy.gauguey@....fr, joel.schopp@....com
Subject: Re: [RFC PATCH 0/2] virtio-mmio: add irqfd support for vhost-net
 based on virtio-mmio

On Sat, Oct 25, 2014 at 04:24:52PM +0800, john.liuli wrote:
> From: Li Liu <john.liuli@...wei.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
> 
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