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
