On Tue, Dec 15, 2015 at 12:40 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > On Mon, Dec 14, 2015 at 10:27:52AM -0800, 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. > > See below. > >> 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? >> >> --Andy > > I think there's more to virtio than just QEMU. > > I have no idea whether anyone implemented hypervisors with an IOMMU. > virtio bypassing iommu makes a lot of sense so it did this since > forever. I do not feel comfortable changing guest/hypervisor ABI and > waiting for people to complain. > > But we do want to fix Xen. > > Let's do this slowly, and whitelist the configurations that > require DMA API to work, so we know we are not breaking anything. > > For example, test a device flag and use iommu if set. > Currently, set it if xen_pv_domain is enabled. > We'll add more as more platforms gain IOMMU support > for virtio and we find ways to identify them. > > It would be kind of a mix of what you did and what Stefano did. > > And alternative would be a quirk: make DMA API create 1:1 mappings for > virtio devices only. Then teach Xen pv to ignore this quirk. This is > what I referred to above. > For example, something like DMA_ATTR_IOMMU_BYPASS would do the trick > nicely. If there's a chance that's going to be upstream, we > could use that. I'd be in favor of that approach, except that apparently PowerPC can't do it (the 1:1 mappings have an offset). I *think* that x86 can do it. I'll re-send the series with DMA API defaulted off except on Xen once the merge window closes. --Andy _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
