RE: [RFC PATCH v1 0/4] vfio: Add IOPF support for VFIO passthrough

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Saturday, January 30, 2021 6:58 AM
> 
> On Mon, 25 Jan 2021 17:03:58 +0800
> Shenming Lu <lushenming@huawei.com> 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. 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.

> 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.

Thanks
Kevin