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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Dec 09, 2020 at 04:26:50AM -0500, Jason Wang wrote:
> ----- Original Message -----
> > 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
> 
> I may miss something but my understanding is that it's the
> responsiblity of device to migrate this part?

Good point. You're right.

> > - 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!
> 
> I think it's a balance. E.g we can poll the virtqueue to have an
> exitless doorbell.
> 
> > 
> > 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.
> 
> Yes. That's our plan. But the interface might require more thought.
> 
> E.g is the bitmap a good approach? To me reporting dirty pages via
> virqueue is better since it get less footprint and is self throttled.
> 
> And we need an address space other than the one used by guest for
> either bitmap for virtqueue.
> 
> > 
> > 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.
> 
> Yes, and actually such slow down may help for the converge of the
> migration.
> 
> Note that the whole idea is try to have a generic solution for all
> types of devices. It's good to consider the performance but for the
> first stage, it should be sufficient to make it work and consider to
> optimize on top.

Moving the shadow vq to the kernel later would be quite a big change
requiring rewriting much of the code. That's why I mentioned this now
before a lot of effort is invested in a QEMU implementation.

> > I just wanted to share the idea of moving the
> > shadow vq into the kernel in case you like that approach better.
> 
> My understanding is to keep kernel as simple as possible and leave the
> polices to userspace as much as possible. E.g it requires us to
> disable doorbell mapping and irq offloading, all of which were under
> the control of userspace.

If the performance is acceptable with the QEMU approach then I think
that's the best place to implement it. It looks high-overhead though so
maybe one of the first things to do is to run benchmarks to collect data
on how it performs?

Stefan

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