Re: [Qemu-devel] Should memory hotplug work with vhost-user backends?

[Raphael Norwitz]

> > I've briefly looked through the libvhost-user code and the hot-add path
> > looks safe enough to me (or at least no more broken than the regular
> > vhost-user memory hot-add path).
> >
> > Can you elaborate a little more about why memory hot-add is unsafe with
> > vhost-user device backends built with libvhost-user, as opposed to those
> > using the raw vhost-user protocol semantics?
>
> The libvhost-user problem is that the library is mostly designed for a
> single-threaded event loop that handles all virtqueue and vhost-user
> protocol activity.
>
> As soon as virtqueues are handled by dedicated threads there are race
> conditions between the virtqueue threads and the vhost-user protocol
> thread.
>
> A virtqueue thread may or may not have an up-to-date view of memory
> translation.  This can result in it missing memory that is currently
> being hotplugged and continuing to access memory that has been removed.
>

I agree - this is definitely seems like a problem if memory is being removed,
but I don’t see how a virtqueue thread may have an outdated view of memory
in the hot-add case.

libvhost-user now supports the REPLY_ACK feature, so that on hot-add qemu
will wait for a response from the backend, confirming the new memory was
successfully mapped in, before returning from vhost_user_set_mem_table(). If
the new memory is mapped in by the backend before the ram is exposed to the
guest, how could a virtqueue thread receive operations on missing memory?

> Dave might have additional comments.
>
> Stefan