> Hi Roman, Hi Julien, >> Also, I added the log in xen_swiotlb_detect() and can see that swiotlb >> still used (other devices within dom0 used too), when dom0 is direct mapped: >> >> [ 1.870363] xen_swiotlb_detect() dev: rcar-fcp, >> XENFEAT_direct_mapped, use swiotlb >> [ 1.878352] xen_swiotlb_detect() dev: rcar-fcp, >> XENFEAT_direct_mapped, use swiotlb >> [ 1.886309] xen_swiotlb_detect() dev: rcar-fcp, >> XENFEAT_direct_mapped, use swiotlb >> >> This means, that all devices are using swiotlb-xen DMA fops. >> By the way, before applying this patches, dom0 always used swiotlb-xen >> fops for initial domain by design. >> This is expected because your domain is direct mapped. May be, I don't understand right, Stefano reported the same issue when dom0 is not direct mapped, but I have direct mapped dom0 and problem still exists. >Ok. Would you be able to provide more information on where the dom0 >memory is allocated and the list of host RAM? Host memory: DRAM: 7.9 GiB Bank #0: 0x048000000 - 0x0bfffffff, 1.9 GiB Bank #1: 0x500000000 - 0x57fffffff, 2 GiB Bank #2: 0x600000000 - 0x67fffffff, 2 GiB Bank #3: 0x700000000 - 0x77fffffff, 2 GiB dom0 memory map: (XEN) Allocating 1:1 mappings totalling 2048MB for dom0: (XEN) BANK[0] 0x00000048000000-0x00000050000000 (128MB) (XEN) BANK[1] 0x00000058000000-0x000000c0000000 (1664MB) (XEN) BANK[2] 0x00000510000000-0x00000520000000 (256MB) >> We retrieved dev_addr(64b1d0000) + size > 32bit mask, but fcp driver >> wants to use only 32 bit boundary address, but that's consequence. >> > Ok. So your device is only capable to do a 32-bit DMA. Is that correct? Yes. >> I think, the main reason of using bounce buffer is MFN address, not DMA >> phys address. >> >I don't understand this sentence. Can you clarify it? This address looks like the MFN because I'm using mapped grant tables from domU. I've added the log and see the following: with swiotlb: [ 78.620386] dma_map_sg_attrs() dev: rcar-du swiotlb, sg_page: fffffe0001b80000, page_to_phy: b6000000, xen_phys_to_dma: 64b1d0000 without swiotlb (worked fine): [ 74.456426] dma_map_sg_attrs() dev: rcar-du direct map, sg_page: fffffe0001b80000, page_to_phy: b6000000, xen_phys_to_dma: b6000000 I guess, need to figure out why we got a normal dom0 DMA address (b6000000) and why 64b1d0000 when using swiotlb. ________________________________ From: Julien Grall Sent: Wednesday, August 11, 2021 1:30 PM To: Roman Skakun ; sstabellini@kernel.org Cc: Bertrand Marquis ; Andrii Anisov ; Volodymyr Babchuk ; Oleksandr Tyshchenko ; Oleksandr Andrushchenko ; xen-devel@lists.xenproject.org ; Roman Skakun ; Jan Beulich Subject: Re: Disable swiotlb for Dom0 On 11/08/2021 09:49, Roman Skakun wrote: > Hi, Julien! Hi Roman, >> > I have observed your patch here: >> >https://urldefense.com/v3/__https://patchwork.kernel.org/project/xen-devel/patch/alpine.DEB.2.21.2102161333090.3234@sstabellini-ThinkPad->>T480s/__;!!GF_29dbcQIUBPA!kH5gzG1mxcIgDqMu2cVjTD3ggN9LiPN4OVinOnqrhLQrNr-mRb72udp2B5XBqZlW$ T480s/__;!!GF_29dbcQIUBPA!kH5gzG1mxcIgDqMu2cVjTD3ggN9LiPN4OVinOnqrhLQrNr-mRb72udp2B5XBqZlW$>[patchwork[.]kernel[.]org] >> > >> > And I collided with the same issue, when Dom0 device trying to use >> > swiotlb fops for devices which are controlled by IOMMU. >> >>The issue Stefano reported was when the dom0 is not direct mapped. >>However... > > I applied these patches: > https://urldefense.com/v3/__https://github.com/torvalds/linux/commit/f5079a9a2a31607a2343e544e9182ce35b030578__;!!GF_29dbcQIUBPA!mZHMTZ8iSAfPg9D1VyO4mamUWVxP7K-H26d1jSf4qMsWkB3l92muGUIJPUgqgI9K$ [github[.]com] > > https://urldefense.com/v3/__https://github.com/xen-project/xen/commit/d66bf122c0ab79063a607d6cf68edf5e91d17d5e__;!!GF_29dbcQIUBPA!mZHMTZ8iSAfPg9D1VyO4mamUWVxP7K-H26d1jSf4qMsWkB3l92muGUIJPcdadGTB$ [github[.]com] > > to check this more pragmatically. > > Also, I added the log in xen_swiotlb_detect() and can see that swiotlb > still used (other devices within dom0 used too), when dom0 is direct mapped: > > [ 1.870363] xen_swiotlb_detect() dev: rcar-fcp, > XENFEAT_direct_mapped, use swiotlb > [ 1.878352] xen_swiotlb_detect() dev: rcar-fcp, > XENFEAT_direct_mapped, use swiotlb > [ 1.886309] xen_swiotlb_detect() dev: rcar-fcp, > XENFEAT_direct_mapped, use swiotlb > > This means, that all devices are using swiotlb-xen DMA fops. > By the way, before applying this patches, dom0 always used swiotlb-xen > fops for initial domain by design. This is expected because your domain is direct mapped. > > >> Any reason to not use the stable branch for 5.10? I don't know whether >> your issue will be fixed there, but the stable branch usually contains a >> lot of bug fixes (including security one). So it is a good idea to use >> it over the first release of a kernel version. > > Yes, sure, current BSP release based on 5.10 kernel: > https://urldefense.com/v3/__https://github.com/xen-troops/linux/tree/v5.10/rcar-5.0.0.rc4-xt0.1__;!!GF_29dbcQIUBPA!mZHMTZ8iSAfPg9D1VyO4mamUWVxP7K-H26d1jSf4qMsWkB3l92muGUIJPVkpyAnu$ [github[.]com] > > based on https://urldefense.com/v3/__https://github.com/renesas-rcar/linux-bsp__;!!GF_29dbcQIUBPA!mZHMTZ8iSAfPg9D1VyO4mamUWVxP7K-H26d1jSf4qMsWkB3l92muGUIJPWPOIbVJ$ [github[.]com] > > BTW, I specified the wrong kernel URL in the previous massage, sorry. > >> > Issue caused in xen_swiotlb_map_page(): >> > ``` >> > dev: rcar-fcp, cap: 0, dma_mask: ffffffff, page: fffffe00180c7400, > page_to_phys: 64b1d0000, >> > xen_phys_to_dma(phys): 64b1d0000 >> > ``` >> >>I can't seem to find this printk in Linux 5.10. Did you add it yourself? > > Yes, it's my own log. Ok. Would you be able to provide more information on where the dom0 memory is allocated and the list of host RAM? > > >>This line suggests that the SWIOTLB tried to bounce the DMA buffer. In >>general, the use of the bounce buffer should be rare. So I would suggest >>to find out why this is used. >> >>Looking at the code, this suggests that one of the following check is false: >> >>/* >> * If the address happens to be in the device's DMA window, >>* we can safely return the device addr and not worry about bounce >>* buffering it. >>*/ >>if (dma_capable(dev, dev_addr, size, true) && >>!range_straddles_page_boundary(phys, size) && >>!xen_arch_need_swiotlb(dev, phys, dev_addr) && >>swiotlb_force != SWIOTLB_FORCE) >>goto done; > > I checked this earlier and saw that dma_capable(dev, dev_addr, size, > true)returns false as expected because > we got dev_addr equals 64b1d0000 and according to this expression under > dma_capable(): > > ``` > dma_addr_t end = dev_addr + size - 1; > return end <= min_not_zero(*dev->dma_mask, dev->bus_dma_limit); > ``` > As result, DMA mask more than 32bit. >> Let me start with that I agree we should disable swiotlb when we know >> the device is protected. However, from what you describe, it sounds like >> the same issue would appear if the IOMMU was disabled. > > Yes, it looks like a potential issue. This means that swiotlb should be > worked correctly, when it's needed, agreed. > But this is also potential improvement, and I presented this idea to > discuss and create some patches. You might be able to remove the Xen swiotlb but I am not sure you will be able to remove the swiotlb completely if you have a device that only supports 32-bit DMA. > >> Therefore, I think we should first find out why Linux wants to bounce >> the DMA buffer. > > We retrieved dev_addr(64b1d0000) + size > 32bit mask, but fcp driver > wants to use only 32 bit boundary address, but that's consequence. Ok. So your device is only capable to do a 32-bit DMA. Is that correct? > I think, the main reason of using bounce buffer is MFN address, not DMA > phys address. I don't understand this sentence. Can you clarify it? Cheers, -- Julien Grall