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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:     Zha Bin <zhabin@linux.alibaba.com>,
        virtio-dev@lists.oasis-open.org, 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
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <d4eb9cde-5d06-3df9-df28-15378a9c6929@redhat.com>
Date:   Wed, 12 Feb 2020 16:53:44 +0800
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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 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 t=
o
>>>> 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


>
> -- MST