On 2021/2/7 16:20, Tian, Kevin wrote: >> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> >> Sent: Friday, February 5, 2021 6:37 PM >> >> Hi, >> >> On Thu, Feb 04, 2021 at 06:52:10AM +0000, Tian, Kevin wrote: >>>>>>> The static pinning and mapping problem in VFIO and possible >> solutions >>>>>>> have been discussed a lot [1, 2]. One of the solutions is to add I/O >>>>>>> page fault support for VFIO devices. Different from those relatively >>>>>>> complicated software approaches such as presenting a vIOMMU that >>>>>> provides >>>>>>> the DMA buffer information (might include para-virtualized >>>> optimizations), >> >> I'm curious about the performance difference between this and the >> map/unmap vIOMMU, as well as the coIOMMU. This is probably a lot faster >> but those don't depend on IOPF which is a pretty rare feature at the >> moment. Yeah, I will give the performance data later. >> >> [...] >>>>> In reality, many >>>>> devices allow I/O faulting only in selective contexts. However, there >>>>> is no standard way (e.g. PCISIG) for the device to report whether >>>>> arbitrary I/O fault is allowed. Then we may have to maintain device >>>>> specific knowledge in software, e.g. in an opt-in table to list devices >>>>> which allows arbitrary faults. For devices which only support selective >>>>> faulting, a mediator (either through vendor extensions on vfio-pci-core >>>>> or a mdev wrapper) might be necessary to help lock down non- >> faultable >>>>> mappings and then enable faulting on the rest mappings. >>>> >>>> For devices which only support selective faulting, they could tell it to the >>>> IOMMU driver and let it filter out non-faultable faults? Do I get it wrong? >>> >>> Not exactly to IOMMU driver. There is already a vfio_pin_pages() for >>> selectively page-pinning. The matter is that 'they' imply some device >>> specific logic to decide which pages must be pinned and such knowledge >>> is outside of VFIO. >>> >>> From enabling p.o.v we could possibly do it in phased approach. First >>> handles devices which tolerate arbitrary DMA faults, and then extends >>> to devices with selective-faulting. The former is simpler, but with one >>> main open whether we want to maintain such device IDs in a static >>> table in VFIO or rely on some hints from other components (e.g. PF >>> driver in VF assignment case). Let's see how Alex thinks about it. >> >> Do you think selective-faulting will be the norm, or only a problem for >> initial IOPF implementations? To me it's the selective-faulting kind of >> device that will be the odd one out, but that's pure speculation. Either >> way maintaining a device list seems like a pain. > > I would think it's norm for quite some time (e.g. multiple years), as from > what I learned turning a complex accelerator to an implementation > tolerating arbitrary DMA fault is way complex (in every critical path) and > not cost effective (tracking in-fly requests). It might be OK for some > purposely-built devices in specific usage but for most it has to be an > evolving path toward the 100%-faultable goal... > >> >> [...] >>> Yes, it's in plan but just not happened yet. We are still focusing on guest >>> SVA part thus only the 1st-level page fault (+Yi/Jacob). It's always >> welcomed >>> to collaborate/help if you have time. 😊 >> >> By the way the current fault report API is missing a way to invalidate >> partial faults: when the IOMMU device's PRI queue overflows, it may >> auto-respond to page request groups that were already partially reported >> by the IOMMU driver. Upon detecting an overflow, the IOMMU driver needs >> to >> tell all fault consumers to discard their partial groups. >> iopf_queue_discard_partial() [1] does this for the internal IOPF handler >> but we have nothing for the lower-level fault handler at the moment. And >> it gets more complicated when injecting IOPFs to guests, we'd need a >> mechanism to recall partial groups all the way through kernel->userspace >> and userspace->guest. > > I didn't know how to recall partial groups through emulated vIOMMUs > (at least for virtual VT-d). Possibly it could be supported by virtio-iommu. > But in any case I consider it more like an optimization instead of a functional > requirement (and could be avoided in below Shenming's suggestion). > >> >> Shenming suggests [2] to also use the IOPF handler for IOPFs managed by >> device drivers. It's worth considering in my opinion because we could hold >> partial groups within the kernel and only report full groups to device >> drivers (and guests). In addition we'd consolidate tracking of IOPFs, >> since they're done both by iommu_report_device_fault() and the IOPF >> handler at the moment. > > I also think it's the right thing to do. In concept w/ or w/o DEV_FEAT_IOPF > just reflects how IOPFs are delivered to the system software. In the end > IOPFs are all about permission violations in the IOMMU page tables thus > we should try to reuse/consolidate the IOMMU fault reporting stack as > much as possible. > >> >> Note that I plan to upstream the IOPF patch [1] as is because it was >> already in good shape for 5.12, and consolidating the fault handler will >> require some thinking. > > This plan makes sense. Yeah, maybe this problem could be left for the implementation of a device(VFIO) specific fault handler... :-) Thanks, Shenming > >> >> Thanks, >> Jean >> >> >> [1] https://lore.kernel.org/linux-iommu/20210127154322.3959196-7-jean- >> philippe@xxxxxxxxxx/ >> [2] https://lore.kernel.org/linux-iommu/f79f06be-e46b-a65a-3951- >> 3e7dbfa66b4a@xxxxxxxxxx/ > > Thanks > Kevin >
