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

From: Eric Auger <eric.auger@linaro.org>
To: Li Liu <john.liuli@huawei.com>, Peter Maydell <peter.maydell@linaro.org>
Cc: Joel Schopp <joel.schopp@amd.com>,
	Yingshiuan Pan <yingshiuan.pan@gmail.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	remy.gauguey@cea.fr, Rusty Russell <rusty@rustcorp.com.au>,
	lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	peter.huangpeng@huawei.com,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	"eric.auger@st.com" <eric.auger@st.com>,
	Christoffer Dall <christoffer.dall@linaro.org>
Subject: Re: [Qemu-devel] [RFC PATCH 0/2] virtio-mmio: add irqfd support for vhost-net based on virtio-mmio
Date: Wed, 05 Nov 2014 09:43:47 +0100	[thread overview]
Message-ID: <5459E343.6030907@linaro.org> (raw)
In-Reply-To: <544E2B18.8010702@huawei.com>

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@huawei.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?

Thank you in advance

Best Regards

Eric

> 
> Li.
>> .
>>
> 
> 