On Thu, Nov 29, 2012 at 03:26:56PM +0100, Stefan Hajnoczi wrote:
> On Thu, Nov 29, 2012 at 03:54:25PM +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.
> > >
> > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> >
> > Worth bothering with binary search?
> > vhost does a linear search over regions because
> > the number of ram regions is very small.
>
> memory.c does binary search. I did the same but in practice there are
> <20 regions for a simple VM. It's probably not worth it but without
> performance results this is speculation.
>
> I think there's no harm in using binary search to start with.
>
> > > +static void hostmem_listener_append_region(MemoryListener *listener,
> > > + MemoryRegionSection *section)
> > > +{
> > > + Hostmem *hostmem = container_of(listener, Hostmem, listener);
> > > +
> > > + if (memory_region_is_ram(section->mr)) {
> > > + hostmem_append_new_region(hostmem, section);
> > > + }
> >
> > I think you also need to remove VGA region since you
> > don't mark these pages as dirty so access there won't work.
>
> I don't understand. If memory in the VGA region returns true from
> memory_region_is_ram(), why would there be a problem?
If you change this memory but you don't update the display.
Never happens with non buggy guests but we should catch and fail if it does.
--
MST
