On 2021/3/18 20:32, Tian, Kevin wrote: >> From: Shenming Lu <lushenming@xxxxxxxxxx> >> Sent: Thursday, March 18, 2021 7:54 PM >> >> On 2021/3/18 17:07, Tian, Kevin wrote: >>>> From: Shenming Lu <lushenming@xxxxxxxxxx> >>>> Sent: Thursday, March 18, 2021 3:53 PM >>>> >>>> On 2021/2/4 14:52, Tian, Kevin wrote:>>> 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. >>>> >>>> Hi Kevin, >>>> >>>> You mentioned selective-faulting some time ago. I still have some doubt >>>> about it: >>>> There is already a vfio_pin_pages() which is used for limiting the IOMMU >>>> group dirty scope to pinned pages, could it also be used for indicating >>>> the faultable scope is limited to the pinned pages and the rest mappings >>>> is non-faultable that should be pinned and mapped immediately? But it >>>> seems to be a little weird and not exactly to what you meant... I will >>>> be grateful if you can help to explain further. :-) >>>> >>> >>> The opposite, i.e. the vendor driver uses vfio_pin_pages to lock down >>> pages that are not faultable (based on its specific knowledge) and then >>> the rest memory becomes faultable. >> >> Ahh... >> Thus, from the perspective of VFIO IOMMU, if IOPF enabled for such device, >> only the page faults within the pinned range are valid in the registered >> iommu fault handler... >> I have another question here, for the IOMMU backed devices, they are >> already >> all pinned and mapped when attaching, is there a need to call >> vfio_pin_pages() >> to lock down pages for them? Did I miss something?... >> > > If a device is marked as supporting I/O page fault (fully or selectively), > there should be no pinning at attach or DMA_MAP time (suppose as > this series does). Then for devices with selective-faulting its vendor > driver will lock down the pages which are not faultable at run-time, > e.g. when intercepting guest registration of a ring buffer... Get it. Thanks a lot for this! :-) Shenming > > Thanks > Kevin >
