> 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... Thanks Kevin
