On Fri, Dec 14, 2012 at 12:45:16PM +0100, Stefan Hajnoczi wrote: > On Wed, Dec 12, 2012 at 4:49 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > > On Wed, Dec 12, 2012 at 04:34:21PM +0100, Stefan Hajnoczi wrote: > >> 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. > > > > Since addresses are aligned this one is fine I think. > > > >> 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 > > > > Yes, this one seems ugly. Maybe add a TODO? > > > > OK let's assume we want to put it in staging/ > > I worry about the virtio-blk changes being isolated. > > Can you put ifdef CONFIG_VIRTIO_BLK_DATA_PLANE around > > them all to avoid dependency on that header completely > > if configured out? > > Okay, I'll move the #ifdefs. I like the stubs in the header file > because it reduces the amount of #ifdefs, but this is easy to change. > > Stefan
Okay. Another option (if you prefer stubs) is to add a stub for access to s->dataplane field, and surround just the field with ifdefs. As it is, this code: if (s->dataplane) { return; } can't be compiled out since compiler is not smart enough to figure out dataplane is never set. -- MST