On Wed, 2008-11-05 at 23:16 +1100, Rusty Russell wrote: > On Wednesday 05 November 2008 09:14:20 Hollis Blanchard wrote: > > Hi Rusty, I'm using a patch that changes the Linux base page size to > > 64K. (This is actually pretty common in ppc64 world, but I happen to be > > trying it on ppc32.) > > > > I'm seeing a problem with virtio. I think at least part of it can be > > explained by qemu's TARGET_PAGE_BITS==12, and the guest's > > PAGE_SHIFT==16. The guest allocates the queue, then passes the pfn (pa > > > > >> PAGE_SHIFT) to the virtio backend (vp_find_vq()). The backend then > > > > calculates the pa as pfn << TARGET_PAGE_BITS. > > > > I have to run right now, but quickly changing qemu TARGET_PAGE_BITS to > > 16 got me a little further but still didn't work. Any thoughts? > > I see Anthony hardwired page size into the queue activation ABI for > virtio_pci. I think that this should be an actual 4096 (or 12) rather than > depending on guest page size: > > virtio_pci.c:247: > iowrite32(virt_to_phys(info->queue) >> PAGE_SHIFT, > vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN); Actually there's an additional complication: PAGE_SIZE is entrenched in the layout of the ring structure itself (large "struct vring" comment in include/linux/virtio_ring.h). Callers of vring_size() demonstrate this problem. I believe the vring is split across two pages so that it could be directly mapped by two untrusted guests: the producer would have RW access to the descriptors and the "available" fields, while the consumer would have only RO access. I don't think this is yet implemented; is it still planned? It looks like vring_size() et al were carefully written to allow arbitrary page sizes, so for now I'll assume that a struct vring that is contained completely within a single guest mapping is OK and work up a patch. It's worth noting that the PFN_SHIFT question (in the other thread) is a separate issue. I could have PFN_SHIFT=10 but define VRING_PAGE_SIZE=4K for internal alignment. I'm a little nervous about making PPC the only arch with VRING_PAGE_SIZE=1K, since that might affect my vring layout in "unusual" ways if it expands beyond 1K but stays inside 4K. -- Hollis Blanchard IBM Linux Technology Center -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
