From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40558) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eMGaC-0006ET-IZ for qemu-devel@nongnu.org; Tue, 05 Dec 2017 12:00:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eMGa8-0002rY-FG for qemu-devel@nongnu.org; Tue, 05 Dec 2017 12:00:36 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43888) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eMGa8-0002rH-6X for qemu-devel@nongnu.org; Tue, 05 Dec 2017 12:00:32 -0500 Date: Tue, 5 Dec 2017 18:00:10 +0100 From: Cornelia Huck Message-ID: <20171205180010.6aad232f.cohuck@redhat.com> In-Reply-To: <20171205184533-mutt-send-email-mst@kernel.org> References: <1512444796-30615-1-git-send-email-wei.w.wang@intel.com> <1512444796-30615-3-git-send-email-wei.w.wang@intel.com> <20171205145950.GF31150@stefanha-x1.localdomain> <20171205174833-mutt-send-email-mst@kernel.org> <20171205164154.GD6712@stefanha-x1.localdomain> <20171205184533-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [virtio-dev] [PATCH v3 2/7] vhost-pci-net: add vhost-pci-net List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Stefan Hajnoczi , virtio-dev@lists.oasis-open.org, avi.cohen@huawei.com, zhiyong.yang@intel.com, jan.kiszka@siemens.com, jasowang@redhat.com, qemu-devel@nongnu.org, Wei Wang , marcandre.lureau@redhat.com, pbonzini@redhat.com On Tue, 5 Dec 2017 18:53:29 +0200 "Michael S. Tsirkin" 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?