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Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pc
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net |
Date: |
Tue, 5 Dec 2017 20:06:57 +0200 |
On Tue, Dec 05, 2017 at 06:00:10PM +0100, Cornelia Huck wrote:
> On Tue, 5 Dec 2017 18:53:29 +0200
> "Michael S. Tsirkin" <address@hidden> 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?
That's just it, I am not entirely sure.
So IMHO it's fine to make it a pci specific thing for now.
virtio started like that too.
--
MST
- [Qemu-devel] [PATCH v3 0/7] Vhost-pci for inter-VM communication, Wei Wang, 2017/12/04
- [Qemu-devel] [PATCH v3 1/7] vhost-user: share the vhost-user protocol related structures, Wei Wang, 2017/12/04
- [Qemu-devel] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Wei Wang, 2017/12/04
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Stefan Hajnoczi, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Michael S. Tsirkin, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Michael S. Tsirkin, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Stefan Hajnoczi, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Michael S. Tsirkin, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Cornelia Huck, 2017/12/05
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Wei Wang, 2017/12/06
- Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net, Stefan Hajnoczi, 2017/12/06
[Qemu-devel] [PATCH v3 3/7] virtio/virtio-pci.c: add vhost-pci-net-pci, Wei Wang, 2017/12/04
[Qemu-devel] [PATCH v3 4/7] vhost-pci-slave: add vhost-pci slave implementation, Wei Wang, 2017/12/04
[Qemu-devel] [PATCH v3 5/7] vhost-user: VHOST_USER_SET_VHOST_PCI msg, Wei Wang, 2017/12/04