Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net

[Michael S. Tsirkin]

On Tue, Dec 05, 2017 at 02:59:50PM +0000, Stefan Hajnoczi wrote:
> On Tue, Dec 05, 2017 at 11:33:11AM +0800, Wei Wang wrote:
> > Add the vhost-pci-net device emulation. The device uses bar 2 to expose
> > the remote VM's memory to the guest. The first 4KB of the the bar area
> > stores the metadata which describes the remote memory and vring info.
> 
> This device looks like the beginning of a new "vhost-pci" virtio device
> type.  There are layering violations:
> 
> 1. This has nothing to do with virtio-net or networking, it's purely
>    vhost-pci.  Why is it called vhost-pci-net instead of vhost-pci?
> 
> 2. VirtIODevice does not know about PCI.  It should work over virtio-ccw
>    or virtio-mmio.  This patch talks about BARs inside a VirtIODevice so
>    there is a problem here.

I think the point is how memory is exposed to another guest.  This
device exposes it as a pci bar. I don't think e.g. ccw can do this,
it's all hypercall-based.

> I'm concerned that there is no clear architecture and elements of the
> virtio architecture are being mixed up with no justification.
> 
> Can you explain what you're trying to do?
> 
> Please post a specification for the vhost-pci device so the operation of
> the device can be discussed and is clear to reviewers.