On Mon, 14 Dec 2015, Andy Lutomirski wrote: > On Mon, Dec 14, 2015 at 6:12 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Mon, Dec 14, 2015 at 02:00:05PM +0000, David Vrabel wrote: > >> On 07/12/15 16:19, Stefano Stabellini wrote: > >> > Hi all, > >> > > >> > this patch series introduces support for running Linux on top of Xen > >> > inside a virtual machine with virtio devices (nested virt scenario). > >> > The problem is that Linux virtio drivers use virt_to_phys to get the > >> > guest pseudo-physical addresses to pass to the backend, which doesn't > >> > work as expected on Xen. > >> > > >> > Switching the virtio drivers to the dma APIs (dma_alloc_coherent, > >> > dma_map/unmap_single and dma_map/unmap_sg) would solve the problem, as > >> > Xen support in Linux provides an implementation of the dma API which > >> > takes care of the additional address conversions. However using the dma > >> > API would increase the complexity of the non-Xen case too. We would also > >> > need to keep track of the physical or virtual address in addition to the > >> > dma address for each vring_desc to be able to free the memory in > >> > detach_buf (see patch #3). > >> > > >> > Instead this series adds few obvious checks to perform address > >> > translations in a couple of key places, without changing non-Xen code > >> > paths. You are welcome to suggest improvements or alternative > >> > implementations. > >> > >> Andy Lutomirski also looked at this. Andy what happened to this work? > >> > >> David > > > > The approach there was to try and convert all virtio to use DMA > > API unconditionally. > > This is reasonable if there's a way for devices to request > > 1:1 mappings individually. > > As that is currently missing, that patchset can not be merged yet. > > > > I still don't understand why *devices* need the ability to request > anything in particular. In current kernels, devices that don't have > an iommu work (and there's no choice about 1:1 or otherwise) and > devices that have an iommu fail spectacularly. With the patches, > devices that don't have an iommu continue to work as long as the DMA > API and/or virtio correctly knows that there's no iommu. Devices that > do have an iommu work fine, albeit slower than would be ideal. In my > book, slower than would be ideal is strictly better than crashing. > > The real issue is *detecting* whether there's an iommu, and the string > of bugs in that area (buggy QEMU for the Q35 thing and complete lack > of a solution for PPC and SPARC is indeed a problem). > > I think that we could apply the series ending here: > > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=virtio_dma&id=ad9d43052da44ce18363c02ea597dde01eeee11b > > and the only regression (performance or functionality) would be that > the buggy Q35 iommu configuration would stop working until someone > fixed it in QEMU. That should be okay -- it's explicitly > experimental. (Xen works with that series applied.) (Actually, > there might be a slight performance regression on PPC due to extra > unused mappings being created. It would be straightforward to hack > around that in one of several ways.) > > Am I missing something? Your changes look plausible and if they fix Xen on virtio I am happy with them. I didn't choose the DMA API approach because, although it looks cleaner, I acknowledge that is a bit invasive. I suggest that the virtio maintainers consider one of the two approaches for inclusion because they fix a real issue. If you would rather avoid the DMA API, then I would be happy to work with you to evolve my current series in a direction of your liking. Please advise on how to proceed. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
