Re: [Qemu-devel] [PATCH 4/5] virtio-blk: release reference to RAM's memoryRegion

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Apr 12, 2013 at 12:48:12PM +0800, liu ping fan wrote:
> On Thu, Apr 11, 2013 at 6:20 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Mon, Apr 01, 2013 at 04:20:33PM +0800, Liu Ping Fan wrote:
> >> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> >>
> >> virtio-blk will reference to RAM's memoryRegion when the req has been
> >> done.  So we can avoid to call bdrv_drain_all() when RAM hot unplug.
> >
> > How does the hot unplug operation work without bdrv_drain_all()?  In
> > other words, how do we safely remove a MemoryRegion and wait for it to
> > become unreferenced?
> >
> bdrv_drain_all() forces the end of usage of memoryRegion. But we can
> let the req done callback ( marks this req finish the end of usage of
> mr) to release the refcnt of memoryRegion.

Yes.  What I'm interested in is the wait mechanism for the QEMU thread
to wait until the memory region(s) become unreferenced.

This patch series is only one half of the memory unplug puzzle and I'd
like to understand how the other half - the unplug operation - will be
implemented.

Just a summary would be interesting - especially how a QEMU thread will
wait until memory regions have been released.  The reference counter
doesn't have any notification that would allow a blocking wait.

Then you have the RCU concept.  So maybe the unplug operation will not
block but instead be called several times from the event loop until all
references have been released?

Stefan