Re: [RFC PATCH 00/27] vDPA software assisted live migration

[Stefan Hajnoczi]

On Fri, Nov 20, 2020 at 07:50:38PM +0100, Eugenio Pérez wrote:
> This series enable vDPA software assisted live migration for vhost-net
> devices. This is a new method of vhost devices migration: Instead of
> relay on vDPA device's dirty logging capability, SW assisted LM
> intercepts dataplane, forwarding the descriptors between VM and device.

Pros:
+ vhost/vDPA devices don't need to implement dirty memory logging
+ Obsoletes ioctl(VHOST_SET_LOG_BASE) and friends

Cons:
- Not generic, relies on vhost-net-specific ioctls
- Doesn't support VIRTIO Shared Memory Regions
  https://github.com/oasis-tcs/virtio-spec/blob/master/shared-mem.tex
- Performance (see below)

I think performance will be significantly lower when the shadow vq is
enabled. Imagine a vDPA device with hardware vq doorbell registers
mapped into the guest so the guest driver can directly kick the device.
When the shadow vq is enabled a vmexit is needed to write to the shadow
vq ioeventfd, then the host kernel scheduler switches to a QEMU thread
to read the ioeventfd, the descriptors are translated, QEMU writes to
the vhost hdev kick fd, the host kernel scheduler switches to the vhost
worker thread, vhost/vDPA notifies the virtqueue, and finally the
vDPA driver writes to the hardware vq doorbell register. That is a lot
of overhead compared to writing to an exitless MMIO register!

If the shadow vq was implemented in drivers/vhost/ and QEMU used the
existing ioctl(VHOST_SET_LOG_BASE) approach, then the overhead would be
reduced to just one set of ioeventfd/irqfd. In other words, the QEMU
dirty memory logging happens asynchronously and isn't in the dataplane.

In addition, hardware that supports dirty memory logging as well as
software vDPA devices could completely eliminate the shadow vq for even
better performance.

But performance is a question of "is it good enough?". Maybe this
approach is okay and users don't expect good performance while dirty
memory logging is enabled. I just wanted to share the idea of moving the
shadow vq into the kernel in case you like that approach better.

Stefan

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