Hi Alex, On 9/29/20 8:18 PM, Alex Williamson wrote: > On Tue, 29 Sep 2020 09:18:22 +0200 > Auger Eric <eric.au...@redhat.com> wrote: > >> Hi all, >> >> [also correcting some outdated email addresses + adding Lorenzo in cc] >> >> On 9/29/20 12:42 AM, Alex Williamson wrote: >>> On Mon, 28 Sep 2020 21:50:34 +0200 >>> Eric Auger <eric.au...@redhat.com> wrote: >>> >>>> VFIO currently exposes the usable IOVA regions through the >>>> VFIO_IOMMU_GET_INFO ioctl / VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE >>>> capability. However it fails to take into account the dma_mask >>>> of the devices within the container. The top limit currently is >>>> defined by the iommu aperture. >>> >>> I think that dma_mask is traditionally a DMA API interface for a device >>> driver to indicate to the DMA layer which mappings are accessible to the >>> device. On the other hand, vfio makes use of the IOMMU API where the >>> driver is in userspace. That userspace driver has full control of the >>> IOVA range of the device, therefore dma_mask is mostly irrelevant to >>> vfio. I think the issue you're trying to tackle is that the IORT code >>> is making use of the dma_mask to try to describe a DMA address >>> limitation imposed by the PCI root bus, living between the endpoint >>> device and the IOMMU. Therefore, if the IORT code is exposing a >>> topology or system imposed device limitation, this seems much more akin >>> to something like an MSI reserved range, where it's not necessarily the >>> device or the IOMMU with the limitation, but something that sits >>> between them. >> >> First I think I failed to explain the context. I worked on NVMe >> passthrough on ARM. The QEMU NVMe backend uses VFIO to program the >> physical device. The IOVA allocator there currently uses an IOVA range >> within [0x10000, 1ULL << 39]. This IOVA layout rather is arbitrary if I >> understand correctly. > > 39 bits is the minimum available on some VT-d systems, so it was > probably considered a reasonable minimum address width to consider. OK > >> I noticed we rapidly get some VFIO MAP DMA >> failures because the allocated IOVA collide with the ARM MSI reserved >> IOVA window [0x8000000, 0x8100000]. Since 9b77e5c79840 ("vfio/type1: >> Check reserved region conflict and update iova list"), such VFIO MAP DMA >> attempts to map IOVAs belonging to host reserved IOVA windows fail. So, >> by using the VFIO_IOMMU_GET_INFO ioctl / >> VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE I can change the IOVA allocator to >> avoid allocating within this range and others. While working on this, I >> tried to automatically compute the min/max IOVAs and change the >> arbitrary [0x10000, 1ULL << 39]. My SMMUv2 supports up to 48b so >> naturally the max IOVA was computed as 1ULL << 48. The QEMU NVMe backend >> allocates at the bottom and at the top of the range. I noticed the use >> case was not working as soon as the top IOVA was more than 1ULL << 42. >> And then we noticed the dma_mask was set to 42 by using >> cat /sys/bus/pci/devices/0005:01:00.0/dma_mask_bits. So my >> interpretation is the dma_mask was somehow containing the info the >> device couldn't handle IOVAs beyond a certain limit. > > I see that there are both OF and ACPI hooks in pci_dma_configure() and > both modify dev->dma_mask, which is what pci-sysfs is exposing here, > but I'm not convinced this even does what it's intended to do. The > driver core calls this via the bus->dma_configure callback before > probing a driver, but then what happens when the driver calls > pci_set_dma_mask()? This is just a wrapper for dma_set_mask() and I > don't see anywhere that would take into account the existing > dev->dma_mask. It seems for example that pci_dma_configure() could > produce a 42 bit mask as we have here, then the driver could override > that with anything that the dma_ops.dma_supported() callback finds > acceptable, and I don't see any instances where the current > dev->dma_mask is considered. Am I overlooking something?
I don't see it either. So the dma_mask set by the driver would never be checked against the dma_mask limited found when parsing OF/ACPI? > >> In my case the 42b limit is computed in iort_dma_setup() by >> acpi_dma_get_range(dev, &dmaaddr, &offset, &size); >> >> Referring to the comment, it does "Evaluate DMA regions and return >> respectively DMA region start, offset and size in dma_addr, offset and >> size on parsing success". This parses the ACPI table, looking for ACPI >> companions with _DMA methods. >> >> But as Alex mentioned, the IORT also allows to define limits on "the >> number of address bits, starting from the least significant bit that can >> be generated by a device when it accesses memory". See Named component >> node.Device Memory Address Size limit or PCI root complex node. Memory >> address size limit. >> >> ret = acpi_dma_get_range(dev, &dmaaddr, &offset, &size); >> if (ret == -ENODEV) >> ret = dev_is_pci(dev) ? rc_dma_get_range(dev, &size) >> : nc_dma_get_range(dev, &size); >> >> So eventually those info collected from the ACPI tables which do impact >> the usable IOVA range seem to be stored in the dma_mask, hence that >> proposal. > > As above, it's not clear to me that anyone other than the driver and > the dma_supported() callback on dma_ops have any input on the value of > dma_mask, so I'm a little baffled by the current operation. me too > >>>> So, for instance, if the IOMMU supports up to 48bits, it may give >>>> the impression the max IOVA is 48b while a device may have a >>>> dma_mask of 42b. So this API cannot really be used to compute >>>> the max usable IOVA. >>>> >>>> This patch removes the IOVA region beyond the dma_mask's. >>> >>> Rather it adds a reserved region accounting for the range above the >>> device's dma_mask. >> >> Yep. It adds new reserved regions in >> /sys/kernel/iommu_groups/<n>/reserved_regions and remove those from the >> usable regions exposed by VFIO GET_INFO. >> >> I don't think the IOMMU API should be consuming >>> dma_mask like this though. For example, what happens in >>> pci_dma_configure() when there are no OF or ACPI DMA restrictions? >> My guess was that the dma_mask was set to the max range but I did not >> test it. > > Still, we're making use of a driver configured property for the > purposes of using the DMA API and consuming it in the IOMMU API, > specifically to satisfy a userspace driver where the in-kernel meta- > driver can't make any assumptions about the device DMA mask. It's all > rather convoluted. I can't object here. Still we are missing missing consolidated info about max IOVA range. I interpreted the dma_mask as the info missing to get it consolidated but now we are not even sure it does the job and even I would hijack its original goal. >> It >>> appears to me that the dma_mask from whatever previous driver had the >>> device carries over to the new driver. That's generally ok for the DMA >>> API because a driver is required to set the device's DMA mask. It >>> doesn't make sense however to blindly consume that dma_mask and export >>> it via an IOMMU API. For example I would expect to see different >>> results depending on whether a host driver has been bound to a device. >>> It seems the correct IOMMU API approach would be for the IORT code to >>> specifically register reserved ranges for the device. >> >> Is it only specific to IORT table? acpi_dma_get_range() in >> drivers/acpi/scan.c is generic. > > Yes, anything trying to implement similar restrictions. It appears to > me that platform code is stepping on a driver owned field used by > dma_ops already here. Maybe reserved regions should be consumed by > dma_ops to understand restrictions between the device and the IOMMU. > >>>> As we start to expose this reserved region in the sysfs file >>>> /sys/kernel/iommu_groups/<n>/reserved_regions, we also need to >>>> handle the IOVA range beyond the IOMMU aperture to handle the case >>>> where the dma_mask would have a higher number of bits than the iommu >>>> max input address. >>> >>> Why? The IOMMU geometry already describes this and vfio combines both >>> the IOMMU geometry and the device reserved regions when generating the >>> IOVA ranges? >> Yes VFIO layer does add the info about the topology but >> /sys/kernel/iommu_groups/<n>/reserved_regions, generated by the IOMMU >> code, does not. this latter only exposes reserved regions. Assume the >> dma_mask is 48b and the IOMMU aperture is 42b (assuming it is possible), >> if you only take into account the "dma_mask" limitation, the end-user >> will interpret this as: I can use up to 48b. > > What end user? Anyone who reads /sys/kernel/iommu_groups/<n>/reserved_regions. A user application could use that info. Anyway I think I indeed wants this file to do more than it was originally meant for, enumerating *some* of the reserved regions. That's a bit frustrating though (same for the VFIO ioctl). The DMA API is allocating within the address space of > the IOMMU, so it will simply never encounter the issue. Within the > IOMMU API we can already query the geometry of the IOMMU to know its > width. It seems like reserved regions is trying to take on new > responsibilities here. > >> Who is going to consume this information? Additionally >>> it appears that reserved regions will report different information >>> depending on whether a device is attached to a domain. >> yes that's correct. Well at some point we decided to expose (some) >> reserved regions through sysfs. Only printing a reduced set of those >> also can be misleading, hence my attempt to be more comprehensive. > > Seems it further blurs the lines of what reserved regions is intended > to report. Is it trying to replace the IOMMU API geometry interface? > >>>> This is a change to the ABI as this reserved region was not yet >>>> exposed in sysfs /sys/kernel/iommu_groups/<n>/reserved_regions or >>>> through the VFIO ioctl. At VFIO level we increment the version of >>>> the VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE capability to advertise >>>> that change. >>> >>> Is this really an ABI change? The original entry for reserved regions >>> includes: >>> >>> Not necessarily all reserved regions are listed. This is typically >>> used to output direct-mapped, MSI, non mappable regions. >> >> I agree. That's not really a change in the ABI but I wanted to make >> things clear about the induced changes for the end-user. On the other >> end there will be a change in the number of reported resv regions. >>> >>> I imagine the intention here was non-mappable relative to the IOMMU, >>> but non-mappable to the device is essentially what we're including >>> here. >>> >>> I'm also concerned about bumping the vfio interface version for the >>> IOVA range. We're not changing the interface, we're modifying the >>> result, and even then only for a fraction of users. How many users are >>> potentially broken by that change? Are we going to bump the version >>> for everyone any time the result changes on any platform? Thanks, >> >> The userspace needs to know if the GET_INFO is reliable to compute the >> min/max IOVAs. If we do not change the version, it cannot know and must >> assume it is not. Wasn't the version field meant for that somehow? > > Our IOVA RANGE is correct to the best of our knowledge, but I don't > think we can or should rev the version every time we find something > incorrect. For instance, we wouldn't do the same for a firmware > induced range that was missing or incorrect. I think the version field > is for creating a new data structure to replace or expand the existing > one, not to simply say the returned data might be different. I > understand the userspace dilemma, but this doesn't seem like the right > solution. Thanks, understood Thanks Eric > > Alex > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu