On Tue, Dec 11, 2012 at 08:09:56PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 11, 2012 at 10:32:28AM -0600, Anthony Liguori wrote: > > "Michael S. Tsirkin" <m...@redhat.com> writes: > > > > > 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 <m...@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 ... > > > > Let's not lose sight of the forest for the trees here... > > > > This whole series is not reusing existing code. That's really the whole > > point. > > > > The point is to take the code (duplication and all) and then do all of > > the refactoring to use common code in the tree itself. > > > > If we want to put this in a hw/staging/ directory, that's fine by me > > too. > > > > Regards, > > > > Anthony Liguori > > Yes I agree. I think lack of handling for cross regin descriptors > bothers me a bit more.
The two things you've mentioned both aren't handled by hw/virtio.c: 1. Issue: Indirect descriptors have no alignment restrictions and can cross regions. hw/virtio.c uses vring_desc_flags() and other accessor functions, which do lduw_phys() - there is no memory region boundary checking here. 2. Issue: Virtio buffers can cross memory region boundaries. hw/virtio.c maps buffers 1:1 using virtqueue_map_sg() and exits if mapping fails. It does not split buffers if they cross a memory region. These are definitely ugly corner cases but hw/virtio.c is proof that we're not hitting them in practice. Stefan
