> From: Shenming Lu <lushenming@xxxxxxxxxx> > Sent: Tuesday, February 2, 2021 2:42 PM > > On 2021/2/1 15:56, Tian, Kevin wrote: > >> From: Alex Williamson <alex.williamson@xxxxxxxxxx> > >> Sent: Saturday, January 30, 2021 6:58 AM > >> > >> On Mon, 25 Jan 2021 17:03:58 +0800 > >> Shenming Lu <lushenming@xxxxxxxxxx> wrote: > >> > >>> Hi, > >>> > >>> 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), > >>> IOPF mainly depends on the hardware faulting capability, such as the > PCIe > >>> PRI extension or Arm SMMU stall model. What's more, the IOPF support > in > >>> the IOMMU driver is being implemented in SVA [3]. So do we consider to > >>> add IOPF support for VFIO passthrough based on the IOPF part of SVA at > >>> present? > >>> > >>> We have implemented a basic demo only for one stage of translation > (GPA > >>> -> HPA in virtualization, note that it can be configured at either stage), > >>> and tested on Hisilicon Kunpeng920 board. The nested mode is more > >> complicated > >>> since VFIO only handles the second stage page faults (same as the non- > >> nested > >>> case), while the first stage page faults need to be further delivered to > >>> the guest, which is being implemented in [4] on ARM. My thought on this > >>> is to report the page faults to VFIO regardless of the occured stage (try > >>> to carry the stage information), and handle respectively according to the > >>> configured mode in VFIO. Or the IOMMU driver might evolve to support > >> more... > >>> > >>> Might TODO: > >>> - Optimize the faulting path, and measure the performance (it might still > >>> be a big issue). > >>> - Add support for PRI. > >>> - Add a MMU notifier to avoid pinning. > >>> - Add support for the nested mode. > >>> ... > >>> > >>> Any comments and suggestions are very welcome. :-) > >> > >> I expect performance to be pretty bad here, the lookup involved per > >> fault is excessive. There are cases where a user is not going to be > >> willing to have a slow ramp up of performance for their devices as they > >> fault in pages, so we might need to considering making this > >> configurable through the vfio interface. Our page mapping also only > > > > There is another factor to be considered. The presence of IOMMU_ > > DEV_FEAT_IOPF just indicates the device capability of triggering I/O > > page fault through the IOMMU, but not exactly means that the device > > can tolerate I/O page fault for arbitrary DMA requests. > > Yes, so I add a iopf_enabled field in VFIO to indicate the whole path faulting > capability and set it to true after registering a VFIO page fault handler. > > > 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. > > > > >> grows here, should mappings expire or do we need a least recently > >> mapped tracker to avoid exceeding the user's locked memory limit? How > >> does a user know what to set for a locked memory limit? The behavior > >> here would lead to cases where an idle system might be ok, but as soon > >> as load increases with more inflight DMA, we start seeing > >> "unpredictable" I/O faults from the user perspective. Seems like there > >> are lots of outstanding considerations and I'd also like to hear from > >> the SVA folks about how this meshes with their work. Thanks, > >> > > > > The main overlap between this feature and SVA is the IOPF reporting > > framework, which currently still has gap to support both in nested > > mode, as discussed here: > > > > https://lore.kernel.org/linux-acpi/YAaxjmJW+ZMvrhac@myrica/ > > > > Once that gap is resolved in the future, the VFIO fault handler just > > adopts different actions according to the fault-level: 1st level faults > > are forwarded to userspace thru the vSVA path while 2nd-level faults > > are fixed (or warned if not intended) by VFIO itself thru the IOMMU > > mapping interface. > > I understand what you mean is: > From the perspective of VFIO, first, we need to set FEAT_IOPF, and then > regster its > own handler with a flag to indicate FLAT or NESTED and which level is > concerned, > thus the VFIO handler can handle the page faults directly according to the > carried > level information. > > Is there any plan for evolving(implementing) the IOMMU driver to support > this? Or > could we help this? :-) > 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. 😊 Thanks Kevin
