Re: [Qemu-devel] [PATCH v4 03/11] dataplane: add host memory mapping code

[Stefan Hajnoczi]

On Thu, Nov 29, 2012 at 02:33:11PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 22, 2012 at 04:16:44PM +0100, Stefan Hajnoczi wrote:
> > The data plane thread needs to map guest physical addresses to host
> > pointers.  Normally this is done with cpu_physical_memory_map() but the
> > function assumes the global mutex is held.  The data plane thread does
> > not touch the global mutex and therefore needs a thread-safe memory
> > mapping mechanism.
> > 
> > Hostmem registers a MemoryListener similar to how vhost collects and
> > pushes memory region information into the kernel.  There is a
> > fine-grained lock on the regions list which is held during lookup and
> > when installing a new regions list.
> > 
> > When the physical memory map changes the MemoryListener callbacks are
> > invoked.  They build up a new list of memory regions which is finally
> > installed when the list has been completed.
> > 
> > Note that this approach is not safe across memory hotplug because mapped
> > pointers may still be in used across memory unplug.  However, this is
> > currently a problem for QEMU in general and needs to be addressed in the
> > future.
> 
> Sounds like a serious problem.
> I'm not sure I understand - do you say this currently a problem for QEMU
> virtio? Coul you give an example please?

This is a limitation of the memory API but cannot be triggered by users
today since we don't support memory hot unplug.  I'm just explaining
that virtio-blk-data-plane has the same issue as hw/virtio-blk.c or any
other device emulation code here.

Some more detail:

The issue is that hw/virtio-blk.c submits an asynchronous I/O request on
the host with the guest buffer.  Then virtio-blk emulation returns back
to the caller and continues QEMU execution.

It is unsafe to unplug memory while the I/O request is pending since
there's no mechanism (e.g. refcount) to wait until the guest memory is
no longer in use.

This is a known issue.  There's no way to trigger a problem today but we
need to eventually enhance QEMU's memory API to handle this case.

Stefan