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[79.182.31.92]) by smtp.gmail.com with ESMTPSA id e8sm26368886wrv.24.2020.06.24.15.16.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Jun 2020 15:16:30 -0700 (PDT) Date: Wed, 24 Jun 2020 18:16:26 -0400 From: "Michael S. Tsirkin" To: Stefano Stabellini Cc: Peng Fan , boris.ostrovsky@oracle.com, jgross@suse.com, konrad.wilk@oracle.com, jasowang@redhat.com, x86@kernel.org, xen-devel@lists.xenproject.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, virtualization@lists.linux-foundation.org, linux-imx@nxp.com Subject: Re: [PATCH] xen: introduce xen_vring_use_dma Message-ID: <20200624181026-mutt-send-email-mst@kernel.org> References: <20200624091732.23944-1-peng.fan@nxp.com> <20200624050355-mutt-send-email-mst@kernel.org> <20200624163940-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > > > > > 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. > it is a detail of > the way Linux does Xen address translations. swiotlb-xen is used to do > these translations and it is hooked into the dma_ops framework. > > It would be possible to have a device in hardware that is > virtio-compatible and doesn't set VIRTIO_F_IOMMU_PLATFORM. That device would be basically broken, since hardware can't know whether it can access all memory or not. > The device > could be directly assigned (passthrough) to a DomU. We would still > have to use swiotlb_xen if Xen is running. > > You should think of swiotlb-xen as only internal to Linux and not > related to whether the (virtual or non-virtual) hardware comes with an > IOMMU or not. IOMMU is a misnomer here. Virtio spec now calls this bit VIRTIO_F_ACCESS_PLATFORM. We should have done the same a while ago - I'll send a patch. > > > > 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. > > Alternatively we could try to work-around it from swiotlb-xen. We could > enable swiotlb-xen for Xen/ARM DomUs with a different implementation so > that we could leave the vring_use_dma_api check unmodified. > > It would be ugly because we would have to figure out from the new > swiotlb-xen functions if the device is a normal device, so we have to > call the regular dma_ops functions, or if the device is a virtio device, > in which case there is nothing to do. I think it is undesirable but > could probably be made to work.