Hi, Julien! Thanks for the answer!
>> 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$<https://urldefense.com/v3/__https://patchwork.kernel.org/project/xen-devel/patch/alpine.DEB.2.21.2102161333090.3234@sstabellini-ThinkPad->T480s/__;!!GF_29dbcQIUBPA!kH5gzG1mxcIgDqMu2cVjTD3ggN9LiPN4OVinOnqrhLQrNr-mRb72udp2B5XBqZlW$> >> [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://github.com/torvalds/linux/commit/f5079a9a2a31607a2343e544e9182ce35b030578 https://github.com/xen-project/xen/commit/d66bf122c0ab79063a607d6cf68edf5e91d17d5e 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. > 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://github.com/xen-troops/linux/tree/v5.10/rcar-5.0.0.rc4-xt0.1 based on https://github.com/renesas-rcar/linux-bsp<https://github.com/renesas-rcar/linux-bsp/tree/v5.10.41/rcar-5.1.0.rc2> 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. > 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. > 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. I think, the main reason of using bounce buffer is MFN address, not DMA phys address. Cheers! ________________________________ From: Julien Grall <[email protected]> Sent: Tuesday, August 10, 2021 7:16 PM To: Roman Skakun <[email protected]>; [email protected] <[email protected]> Cc: Bertrand Marquis <[email protected]>; Andrii Anisov <[email protected]>; Volodymyr Babchuk <[email protected]>; Oleksandr Tyshchenko <[email protected]>; Oleksandr Andrushchenko <[email protected]>; [email protected] <[email protected]>; Roman Skakun <[email protected]>; Jan Beulich <[email protected]> Subject: Re: Disable swiotlb for Dom0 On 10/08/2021 16:38, Roman Skakun wrote: > Hi, Stefano! Hi, > 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$ > [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... > > Prerequisites: > https://urldefense.com/v3/__https://github.com/xen-project/xen/tree/stable-4.15__;!!GF_29dbcQIUBPA!kH5gzG1mxcIgDqMu2cVjTD3ggN9LiPN4OVinOnqrhLQrNr-mRb72udp2B6EVrRb_$ > [github[.]com] ... if you are really using 4.15, then the domain will always be direct mapped. So I think this is a different one. > https://urldefense.com/v3/__https://github.com/torvalds/linux/tree/v5.10__;!!GF_29dbcQIUBPA!kH5gzG1mxcIgDqMu2cVjTD3ggN9LiPN4OVinOnqrhLQrNr-mRb72udp2B1n8el4F$ > [github[.]com] 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. > 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? > > There is retrieved MFN(0x64b1d0000), which belongs to DomU. Dom0 > swiotlb couldn't proceed to this address and throws the log: > > ``` > [ 99.504990] rcar-fcp fea2f000.fcp: swiotlb buffer is full (sz: 3686400 > bytes), total 32768 (slots), used 64 (slots) > ``` 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; > > Temporary, I resolved this issue by disabling swiotlb for dom0 at all > because sure that all devices goes through IOMMU, but this mention can > be true only for me. > > But, I think of a more reliable way is to declare a special IOMMU > property in xen dts for each device. If the device controlled by IOMMU > not need to set swiotlb fops in arch_setup_dma_ops. > What do you think about it? 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. Therefore, I think we should first find out why Linux wants to bounce the DMA buffer. Does your device have any DMA restriction? Cheers, -- Julien Grall
