linux-kernel - Re: [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue

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Date:   Wed, 12 Feb 2020 17:33:06 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Zha Bin <zhabin@...ux.alibaba.com>,
        virtio-dev@...ts.oasis-open.org, slp@...hat.com,
        jing2.liu@...ux.intel.com, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, chao.p.peng@...ux.intel.com,
        gerry@...ux.alibaba.com
Subject: Re: [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue


On 2020/2/12 下午4:53, Jason Wang wrote:
>
> On 2020/2/12 下午4:18, Michael S. Tsirkin wrote:
>> On Wed, Feb 12, 2020 at 11:39:54AM +0800, Jason Wang wrote:
>>> On 2020/2/11 下午7:33, Michael S. Tsirkin wrote:
>>>> On Mon, Feb 10, 2020 at 05:05:17PM +0800, Zha Bin wrote:
>>>>> From: Liu Jiang<gerry@...ux.alibaba.com>
>>>>>
>>>>> The standard virtio-mmio devices use notification register to signal
>>>>> backend. This will cause vmexits and slow down the performance 
>>>>> when we
>>>>> passthrough the virtio-mmio devices to guest virtual machines.
>>>>> We proposed to update virtio over MMIO spec to add the per-queue
>>>>> notify feature VIRTIO_F_MMIO_NOTIFICATION[1]. It can allow the VMM to
>>>>> configure notify location for each queue.
>>>>>
>>>>> [1]https://lkml.org/lkml/2020/1/21/31
>>>>>
>>>>> Signed-off-by: Liu Jiang<gerry@...ux.alibaba.com>
>>>>> Co-developed-by: Zha Bin<zhabin@...ux.alibaba.com>
>>>>> Signed-off-by: Zha Bin<zhabin@...ux.alibaba.com>
>>>>> Co-developed-by: Jing Liu<jing2.liu@...ux.intel.com>
>>>>> Signed-off-by: Jing Liu<jing2.liu@...ux.intel.com>
>>>>> Co-developed-by: Chao Peng<chao.p.peng@...ux.intel.com>
>>>>> Signed-off-by: Chao Peng<chao.p.peng@...ux.intel.com>
>>>> Hmm. Any way to make this static so we don't need
>>>> base and multiplier?
>>> E.g page per vq?
>>>
>>> Thanks
>> Problem is, is page size well defined enough?
>> Are there cases where guest and host page sizes differ?
>> I suspect there might be.
>
>
> Right, so it looks better to keep base and multiplier, e.g for vDPA.
>
>
>>
>> But I also think this whole patch is unproven. Is someone actually
>> working on QEMU code to support pass-trough of virtio-pci
>> as virtio-mmio for nested guests? What's the performance
>> gain like?
>
>
> I don't know.
>
> Thanks


Btw, I think there's no need for a nested environment to test. Current 
eventfd hook to MSIX should still work for MMIO.

Thanks