Re: [Qemu-devel] [PATCH v6 03/12] dataplane: add host memory mapping code

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 11, 2012 at 04:27:49PM +0100, Stefan Hajnoczi wrote:
> On Tue, Dec 11, 2012 at 3:13 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Mon, Dec 10, 2012 at 02:09:36PM +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.
> >
> > Can we export and reuse the vhost code for this?
> > I think you will find this advantageous when you add migration
> > support down the line.
> > And if you find it necessary to use MemoryListener e.g. for performance
> > reasons, then vhost will likely benefit too.
> 
> It's technically possible and not hard to do but it prevents
> integrating deeper with core QEMU as the memory API becomes
> thread-safe.
> 
> There are two ways to implement dirty logging:
> 1. The vhost log approach which syncs dirty information periodically.
> 2. A cheap thread-safe way to mark dirty outside the global mutex,
> i.e. a thread-safe memory_region_set_dirty().

You don't normally want to dirty the whole region,
you want to do this to individual pages.

> If we can get thread-safe guest memory load/store in QEMU then #2 is
> included.  We can switch to using hw/virtio.c instead of
> hw/dataplane/vring.c, we get dirty logging for free, we can drop
> hostmem.c completely, etc.
> 
> Stefan

So why not reuse existing code? If you drop it later it won't
matter what you used ...

-- 
MST