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Date:   Wed, 12 Feb 2020 04:55:38 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@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
Subject: Re: [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue
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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@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 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.

-- 
MST