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Subject: Re: [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtio-dev@lists.oasis-open.org,
        Zha Bin <zhabin@linux.alibaba.com>, slp@redhat.com,
        jing2.liu@linux.intel.com, linux-kernel@vger.kernel.org,
        qemu-devel@nongnu.org, chao.p.peng@linux.intel.com,
        gerry@linux.alibaba.com
References: <cover.1581305609.git.zhabin@linux.alibaba.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <ff1a4d50-cfff-a354-3c64-30a6ed50a9fe@redhat.com>
Date:   Thu, 13 Feb 2020 11:38:31 +0800
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On 2020/2/12 =E4=B8=8B=E5=8D=885:55, Michael S. Tsirkin wrote:
> On Wed, Feb 12, 2020 at 05:33:06PM +0800, Jason Wang wrote:
>> On 2020/2/12 =E4=B8=8B=E5=8D=884:53, Jason Wang wrote:
>>> On 2020/2/12 =E4=B8=8B=E5=8D=884:18, Michael S. Tsirkin wrote:
>>>> On Wed, Feb 12, 2020 at 11:39:54AM +0800, Jason Wang wrote:
>>>>> On 2020/2/11 =E4=B8=8B=E5=8D=887:33, Michael S. Tsirkin wrote:
>>>>>> On Mon, Feb 10, 2020 at 05:05:17PM +0800, Zha Bin wrote:
>>>>>>> From: Liu Jiang<gerry@linux.alibaba.com>
>>>>>>>
>>>>>>> The standard virtio-mmio devices use notification register to sig=
nal
>>>>>>> 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 VM=
M to
>>>>>>> configure notify location for each queue.
>>>>>>>
>>>>>>> [1]https://lkml.org/lkml/2020/1/21/31
>>>>>>>
>>>>>>> Signed-off-by: Liu Jiang<gerry@linux.alibaba.com>
>>>>>>> Co-developed-by: Zha Bin<zhabin@linux.alibaba.com>
>>>>>>> Signed-off-by: Zha Bin<zhabin@linux.alibaba.com>
>>>>>>> Co-developed-by: Jing Liu<jing2.liu@linux.intel.com>
>>>>>>> Signed-off-by: Jing Liu<jing2.liu@linux.intel.com>
>>>>>>> Co-developed-by: Chao Peng<chao.p.peng@linux.intel.com>
>>>>>>> Signed-off-by: Chao Peng<chao.p.peng@linux.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=20
was measured by vhost or other which needs some clarification.

Thanks


>