Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

[Michael S. Tsirkin]

On Thu, Dec 07, 2017 at 11:57:33AM +0800, Wei Wang wrote:
> On 12/07/2017 12:27 AM, Stefan Hajnoczi wrote:
> > On Wed, Dec 6, 2017 at 4:09 PM, Wang, Wei W <address@hidden> 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.
> 
> I intend to have a different opinion about this:
> 
> 1) Even relaying the msgs to the guest, QEMU still need to handle the msg
> first, for example, it needs to decode the msg to see if it is the ones
> (e.g. SET_MEM_TABLE, SET_VRING_KICK, SET_VRING_CALL) that should be used for
> the device setup (e.g. mmap the memory given via SET_MEM_TABLE). In this
> case, we will be likely to have 2 slave handlers - one in the guest, another
> in QEMU device.
> 
> 2) If people already understand the vhost-user protocol, it would be natural
> for them to understand the vhost-pci metadata - just the obtained memory and
> vring info are put to the metadata area (no new things).

I see a bigger problem with passthrough. If qemu can't fully decode all
messages, it can not operate in a disconected mode - guest will have to
stop on disconnect until we re-connect a backend.

> 
> Inspired from your sharing, how about the following:
> we can actually factor out a common vhost-pci layer, which handles all the
> features that are common to all the vhost-pci series of devices
> (vhost-pci-net, vhost-pci-blk,...)
> Coming to the implementation, we can have a VhostpciDeviceClass (similar to
> VirtioDeviceClass), the device realize sequence will be 
> virtio_device_realize()-->vhost_pci_device_realize()-->vhost_pci_net_device_realize()
> 
> 
> 
> > 
> > > > 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/address@hidden/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.
> 
> 
> Yes, that tx is a dummy actually, just created to use its doorbell.
> Currently, with virtio_device, I think ioeventfd comes with virtqueue only.
> Actually, I think we could have the issues solved by vhost-pci. For example,
> reserve a piece of  the BAR area for ioeventfd. The bar layout can be:
> BAR 2:
> 0~4k: vhost-pci device specific usages (ioeventfd etc)
> 4k~8k: metadata (memory info and vring info)
> 8k~64GB: remote guest memory
> (we can make the bar size (64GB is the default value used) configurable via
> qemu cmdline)
> 
> 
> Best,
> Wei
> 
> 
> 
> 
>