On Thu, Nov 29, 2012 at 02:57:05PM +0200, Michael S. Tsirkin wrote: > On Thu, Nov 29, 2012 at 02:54:26PM +0200, Michael S. Tsirkin wrote: > > On Thu, Nov 29, 2012 at 01:45:19PM +0100, Stefan Hajnoczi wrote: > > > 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 > > > > For this problem we would simply need to flush outstanding aio > > before freeing memory for unplug, no refcount necessary. > > > > This patch however introduces the issue in the frontend > > and it looks like there won't be any way to solve > > it without changing the API. > > To clarify, as you say it is not triggerable > so I don't think this is strictly required to address > this at this point though it should not be too hard: > just register callback that flushes the frontend processing. > > But if you can't code it at this point, please add > a TODO comment in code.
Yes, I'm adding a TODO and your suggestion to flush the frontend sounds like a simple solution - we already quiesce at other critical points like live migration. Stefan
