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

[Stefan Hajnoczi]

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.

>> 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.