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

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Date:   Thu, 13 Feb 2020 11:38:31 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtio-dev@...ts.oasis-open.org,
        Zha Bin <zhabin@...ux.alibaba.com>, 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5:55, Michael S. Tsirkin wrote:
> On Wed, Feb 12, 2020 at 05:33:06PM +0800, Jason Wang wrote:
>> 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
>
> Oh yes it's the wildcard thingy but how much extra performance does one get
> from it with MMIO? A couple % might not be worth the trouble for MMIO.


The cover letter have some numbers but I'm not sure whether or not it 
was measured by vhost or other which needs some clarification.

Thanks


>