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

[Tian, Kevin]

> 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.

Thanks
Kevin