On Thu, Jun 25, 2020 at 10:31:27AM -0700, Stefano Stabellini wrote: > On Wed, 24 Jun 2020, Michael S. Tsirkin wrote: > > On Wed, Jun 24, 2020 at 02:53:54PM -0700, Stefano Stabellini wrote: > > > On Wed, 24 Jun 2020, Michael S. Tsirkin wrote: > > > > On Wed, Jun 24, 2020 at 10:59:47AM -0700, Stefano Stabellini wrote: > > > > > On Wed, 24 Jun 2020, Michael S. Tsirkin wrote: > > > > > > On Wed, Jun 24, 2020 at 05:17:32PM +0800, Peng Fan wrote: > > > > > > > Export xen_swiotlb for all platforms using xen swiotlb > > > > > > > > > > > > > > Use xen_swiotlb to determine when vring should use dma APIs to map the > > > > > > > ring: when xen_swiotlb is enabled the dma API is required. When it is > > > > > > > disabled, it is not required. > > > > > > > > > > > > > > Signed-off-by: Peng Fan <peng.fan@xxxxxxx> > > > > > > > > > > > > Isn't there some way to use VIRTIO_F_IOMMU_PLATFORM for this? > > > > > > Xen was there first, but everyone else is using that now. > > > > > > > > > > Unfortunately it is complicated and it is not related to > > > > > VIRTIO_F_IOMMU_PLATFORM :-( > > > > > > > > > > > > > > > The Xen subsystem in Linux uses dma_ops via swiotlb_xen to translate > > > > > foreign mappings (memory coming from other VMs) to physical addresses. > > > > > On x86, it also uses dma_ops to translate Linux's idea of a physical > > > > > address into a real physical address (this is unneeded on ARM.) > > > > > > > > > > > > > > > So regardless of VIRTIO_F_IOMMU_PLATFORM, dma_ops should be used on Xen/x86 > > > > > always and on Xen/ARM if Linux is Dom0 (because it has foreign > > > > > mappings.) That is why we have the if (xen_domain) return true; in > > > > > vring_use_dma_api. > > > > > > > > VIRTIO_F_IOMMU_PLATFORM makes guest always use DMA ops. > > > > > > > > Xen hack predates VIRTIO_F_IOMMU_PLATFORM so it *also* > > > > forces DMA ops even if VIRTIO_F_IOMMU_PLATFORM is clear. > > > > > > > > Unfortunately as a result Xen never got around to > > > > properly setting VIRTIO_F_IOMMU_PLATFORM. > > > > > > I don't think VIRTIO_F_IOMMU_PLATFORM would be correct for this because > > > the usage of swiotlb_xen is not a property of virtio, > > > > > > Basically any device without VIRTIO_F_ACCESS_PLATFORM > > (that is it's name in latest virtio spec, VIRTIO_F_IOMMU_PLATFORM is > > what linux calls it) is declared as "special, don't follow normal rules > > for access". > > > > So yes swiotlb_xen is not a property of virtio, but what *is* a property > > of virtio is that it's not special, just a regular device from DMA POV. > > I am trying to understand what you meant but I think I am missing > something. > > Are you saying that modern virtio should always have > VIRTIO_F_ACCESS_PLATFORM, hence use normal dma_ops as any other devices? I am saying it's a safe default. Clear VIRTIO_F_ACCESS_PLATFORM if you have some special needs e.g. you are very sure it's ok to bypass DMA ops, or you need to support a legacy guest (produced in the window between virtio 1 support in 2014 and support for VIRTIO_F_ACCESS_PLATFORM in 2016). > If that is the case, how is it possible that virtio breaks on ARM using > the default dma_ops? The breakage is not Xen related (except that Xen > turns dma_ops on). The original message from Peng was: > > vring_map_one_sg -> vring_use_dma_api > -> dma_map_page > -> __swiotlb_map_page > ->swiotlb_map_page > ->__dma_map_area(phys_to_virt(dma_to_phys(dev, dev_addr)), size, dir); > However we are using per device dma area for rpmsg, phys_to_virt > could not return a correct virtual address for virtual address in > vmalloc area. Then kernel panic. > > I must be missing something. Maybe it is because it has to do with RPMesg? I think it's an RPMesg bug, yes. > > > > > > You might have noticed that I missed one possible case above: Xen/ARM > > > > > DomU :-) > > > > > > > > > > Xen/ARM domUs don't need swiotlb_xen, it is not even initialized. So if > > > > > (xen_domain) return true; would give the wrong answer in that case. > > > > > Linux would end up calling the "normal" dma_ops, not swiotlb-xen, and > > > > > the "normal" dma_ops fail. > > > > > > > > > > > > > > > The solution I suggested was to make the check in vring_use_dma_api more > > > > > flexible by returning true if the swiotlb_xen is supposed to be used, > > > > > not in general for all Xen domains, because that is what the check was > > > > > really meant to do. > > > > > > > > Why not fix DMA ops so they DTRT (nop) on Xen/ARM DomU? What is wrong with that? > > > > > > swiotlb-xen is not used on Xen/ARM DomU, the default dma_ops are the > > > ones that are used. So you are saying, why don't we fix the default > > > dma_ops to work with virtio? > > > > > > It is bad that the default dma_ops crash with virtio, so yes I think it > > > would be good to fix that. However, even if we fixed that, the if > > > (xen_domain()) check in vring_use_dma_api is still a problem. > > > > Why is it a problem? It just makes virtio use DMA API. > > If that in turn works, problem solved. > > You are correct in the sense that it would work. However I do think it > is wrong for vring_use_dma_api to enable dma_ops/swiotlb-xen for Xen/ARM > DomUs that don't need it. There are many different types of Xen guests, > Xen x86 is drastically different from Xen ARM, it seems wrong to treat > them the same way. I could imagine some future Xen hosts setting a flag somewhere in the platform capability saying "no xen specific flag, rely on "VIRTIO_F_ACCESS_PLATFORM". Then you set that accordingly in QEMU. How about that? > > > Anyway, re-reading the last messages of the original thread [1], it > looks like Peng had a clear idea on how to fix the general issue. Peng, > what happened with that? > > > [1] https://lore.kernel.org/patchwork/patch/1033801/#1222404 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
