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

[Jason Wang]

On 2020/2/12 äå4:18, Michael S. Tsirkin wrote:
> On Wed, Feb 12, 2020 at 11:39:54AM +0800, Jason Wang wrote:
> > On 2020/2/11 äå7:33, Michael S. Tsirkin wrote:
> > > On Mon, Feb 10, 2020 at 05:05:17PM +0800, Zha Bin wrote:
> > > > From: Liu Jiang<gerry@xxxxxxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxxxxxxx>
> > > > Co-developed-by: Zha Bin<zhabin@xxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Zha Bin<zhabin@xxxxxxxxxxxxxxxxx>
> > > > Co-developed-by: Jing Liu<jing2.liu@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Jing Liu<jing2.liu@xxxxxxxxxxxxxxx>
> > > > Co-developed-by: Chao Peng<chao.p.peng@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Chao Peng<chao.p.peng@xxxxxxxxxxxxxxx>
> > > 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