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

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Message-ID: <20200212024158-mutt-send-email-mst@kernel.org>
Date:   Wed, 12 Feb 2020 03:18:57 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Zha Bin <zhabin@...ux.alibaba.com>,
        virtio-dev@...ts.oasis-open.org, 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 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.

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?

-- 
MST