On Mon, Jun 29, 2020 at 03:05:19AM +0000, Peng Fan wrote: > > Subject: Re: [PATCH] xen: introduce xen_vring_use_dma > > > > 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 > > rpmsg bug is another issue, it should not use dma_alloc_coherent for reserved area, > and use vmalloc_to_page. > > Anyway here using dma api will also trigger issue. > > > > > > > > > > > > > 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? > > > > Michael, Stefano, > > So what's your suggestion here, that we could avoid similar issue > for virtio drivers in ARM DomU? > > Thanks, > Peng. > > > > > > > > > > > > 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? > > We shrinked the rpmsg reserved area to workaround the issue. > So still use the dma apis in rpmsg. > > But here I am going to address domu android trusty issue using > virtio. My suggestion is to first of all fix DMA API so it works properly. > > > > > > > > > [1] > > > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore > > > .kernel.org%2Fpatchwork%2Fpatch%2F1033801%2F%231222404&dat > > a=02%7C0 > > > > > 1%7Cpeng.fan%40nxp.com%7C08ba48d3b3d54e775a8108d819e62fd0%7C68 > > 6ea1d3bc > > > > > 2b4c6fa92cd99c5c301635%7C0%7C0%7C637287823721994475&sdata > > =Cw4FHWrH > > > uVKBCn3%2BKS2VM7cWuGoTI6R7SHJrJSLY5Io%3D&reserved=0 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
