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

[Stefan Hajnoczi <stefanha@gmail.com>]

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 <stefanha@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?