On Wed, Dec 6, 2017 at 4:09 PM, Wang, Wei W <wei.w.w...@intel.com> wrote: > On Wednesday, December 6, 2017 9:50 PM, Stefan Hajnoczi wrote: >> On Tue, Dec 05, 2017 at 11:33:09AM +0800, Wei Wang wrote: >> > Vhost-pci is a point-to-point based inter-VM communication solution. >> > This patch series implements the vhost-pci-net device setup and >> > emulation. The device is implemented as a virtio device, and it is set >> > up via the vhost-user protocol to get the neessary info (e.g the >> > memory info of the remote VM, vring info). >> > >> > Currently, only the fundamental functions are implemented. More >> > features, such as MQ and live migration, will be updated in the future. >> > >> > The DPDK PMD of vhost-pci has been posted to the dpdk mailinglist here: >> > http://dpdk.org/ml/archives/dev/2017-November/082615.html >> >> I have asked questions about the scope of this feature. In particular, I >> think >> it's best to support all device types rather than just virtio-net. Here is a >> design document that shows how this can be achieved. >> >> What I'm proposing is different from the current approach: >> 1. It's a PCI adapter (see below for justification) 2. The vhost-user >> protocol is >> exposed by the device (not handled 100% in >> QEMU). Ultimately I think your approach would also need to do this. >> >> I'm not implementing this and not asking you to implement it. Let's just use >> this for discussion so we can figure out what the final vhost-pci will look >> like. >> >> Please let me know what you think, Wei, Michael, and others. >> > > Thanks for sharing the thoughts. If I understand it correctly, the key > difference is that this approach tries to relay every vhost-user msg to the > guest. I'm not sure about the benefits of doing this. > To make data plane (i.e. driver to send/receive packets) work, I think, > mostly, the memory info and vring info are enough. Other things like callfd, > kickfd don't need to be sent to the guest, they are needed by QEMU only for > the eventfd and irqfd setup.
Handling the vhost-user protocol inside QEMU and exposing a different interface to the guest makes the interface device-specific. This will cause extra work to support new devices (vhost-user-scsi, vhost-user-blk). It also makes development harder because you might have to learn 3 separate specifications to debug the system (virtio, vhost-user, vhost-pci-net). If vhost-user is mapped to a PCI device then these issues are solved. >> vhost-pci is a PCI adapter instead of a virtio device to allow doorbells and >> interrupts to be connected to the virtio device in the master VM in the most >> efficient way possible. This means the Vring call doorbell can be an >> ioeventfd that signals an irqfd inside the host kernel without host userspace >> involvement. The Vring kick interrupt can be an irqfd that is signalled by >> the >> master VM's virtqueue ioeventfd. >> > > > This looks the same as the implementation of inter-VM notification in v2: > https://www.mail-archive.com/qemu-devel@nongnu.org/msg450005.html > which is fig. 4 here: > https://github.com/wei-w-wang/vhost-pci-discussion/blob/master/vhost-pci-rfc2.0.pdf > > When the vhost-pci driver kicks its tx, the host signals the irqfd of > virtio-net's rx. I think this has already bypassed the host userspace (thanks > to the fast mmio implementation) Yes, I think the irqfd <-> ioeventfd mapping is good. Perhaps it even makes sense to implement a special fused_irq_ioevent_fd in the host kernel to bypass the need for a kernel thread to read the eventfd so that an interrupt can be injected (i.e. to make the operation synchronous). Is the tx virtqueue in your inter-VM notification v2 series a real virtqueue that gets used? Or is it just a dummy virtqueue that you're using for the ioeventfd doorbell? It looks like vpnet_handle_vq() is empty so it's really just a dummy. The actual virtqueue is in the vhost-user master guest memory.
