Linux Virtualization - Oct 2014 - [Qemu-devel] [RFC PATCH 0/2] virtio-mmio: add irqfd support for vhost-net based on virtio-mmio

On 25 October 2014 09:24, john.liuli <john.liuli at 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

On 27 October 2014 11:23, Li Liu <john.liuli at huawei.com>
wrote:
> So you mean virtio-mmio will be replaced by PCI/PCIe on ARM at last?
That is the plan, yes. I can't make any promises on
timescales at the moment, though...

-- PMM

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 at 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.
>> .
>>
> 
>