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Subject: Re: [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net
On Tue, Dec 05, 2017 at 04:41:54PM +0000, Stefan Hajnoczi wrote: > On Tue, Dec 05, 2017 at 05:55:45PM +0200, Michael S. Tsirkin wrote: > > 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. > > Yes, that's why the BAR issue needs to be discussed. > > In terms of the patches, the clean way to do it is for the > vhost-pci device to have a memory region that is not called "BAR". The > virtio-pci transport can expose it as a BAR but the device doesn't need > to know about it. Other transports that support memory mapping could > then work with this device too. True, though mmio is pretty much a legacy transport at this point at least from qemu perspective as arm devs don't seem to be working on virtio 1.0 support in qemu. So I am not sure how much of a priority should transport isolation be. > The VIRTIO specification needs to capture this transport requirement > somehow too so it's clear that the vhost device can only run over > transports that support memory mapping. > > That said, it's not clear to me why the vhost-pci device is a VIRTIO > device. It doesn't use virtqueues or the configuration space. It only > uses the vhost-user chardev and the mapped memory. Isn't it better to > make it a PCI device? > > Stefan Seems similar enough to me, except The roles of device and driver are reversed here. -- MST
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