On Tue, 5 Dec 2017 18:53:29 +0200 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> 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. I currently don't see an easy way to make this work via ccw, FWIW. We would need a dedicated mechanism for it, and I'm not sure what the gain would 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. > But will anything other than pci ever make use of this?
