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

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Date:	Thu, 6 Nov 2014 09:59:24 +0800
From:	Shannon Zhao <zhaoshenglong@...wei.com>
To:	Eric Auger <eric.auger@...aro.org>, Li Liu <john.liuli@...wei.com>,
	"Peter Maydell" <peter.maydell@...aro.org>
CC:	Joel Schopp <joel.schopp@....com>,
	Yingshiuan Pan <yingshiuan.pan@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, <remy.gauguey@....fr>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"lkml - Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	Nikolay Nikolaev <n.nikolaev@...tualopensystems.com>,
	QEMU Developers <qemu-devel@...gnu.org>,
	<peter.huangpeng@...wei.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"eric.auger@...com" <eric.auger@...com>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [Qemu-devel] [RFC PATCH 0/2] virtio-mmio: add irqfd support for
 vhost-net based on virtio-mmio



On 2014/11/5 16:43, Eric Auger wrote:
> On 10/27/2014 12:23 PM, Li Liu wrote:
>>
>>
>> On 2014/10/27 17:37, Peter Maydell wrote:
>>> On 25 October 2014 09:24, john.liuli <john.liuli@...wei.com> wrote:
>>>> 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.
>>>
>>> If you want to add a new register you should probably propose
>>> an update to the virtio spec. However, it seems to me it would
>>> be better to get generic PCI/PCIe working on the ARM virt
>>> board instead; then we can let virtio-mmio quietly fade away.
>>> This has been on the todo list for ages (and there have been
>>> RFC patches posted for plain PCI), it's just nobody's had time
>>> to work on it.
>>>
>>> thanks
>>> -- PMM
>>>
>>
>> So you mean virtio-mmio will be replaced by PCI/PCIe on ARM at last?
>> If so, let this patch go with the wind:). Thx.
> 
> Hi,
> 
> As a fix of current situation where ISR is only partially updated when
> vhost-irqfd handles standard IRQ and waiting for PCI emuluation,
> wouldn't it make sense to store ISR content on vhost driver side and
> introduce ioctls to read/write it. When using vhost BE, virtio QEMU
> device would use those ioctl to read/update the ISR content. On top of
> that we would update the ISR in vhost before triggering the irqfd. If I
> do not miss anything this would at least make things functional with irqfd.
> 
> As a second step, we could try to introduce in-kernel emulation of
> ISR/ACK to fix the performance issue related to going to user-side each
> time ISR/ACK accesses are done.
> 
> Do you think it is worth investigating this direction?
> 
Hi,

About this problem I have a talk with Li Liu. As MST said, we could use
multiple GSI to support vhost-net with irqfd. And we have figured out a way
to solve this problem. The method is as same as virtio-pci which is to assign
multiple irqs for virtio-mmio. Also it can support multiqueue virtio-net on arm.

Would you have a look at this method? Thank you very much.

- virtio-mmio: support for multiple irqs
http://www.spinics.net/lists/kernel/msg1858860.html

Thanks,
Shannon

> Thank you in advance
> 
> Best Regards
> 
> Eric
> 
> 
>>
>> Li.
>>> .
>>>

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