Re: [PATCH v5 7/9] iommufd: Associate fault object with iommufd_hw_pgtable

[Baolu Lu]

On 5/27/24 9:33 AM, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxx>
> > Sent: Friday, May 24, 2024 10:25 PM
> >
> > On Mon, May 20, 2024 at 03:39:54AM +0000, Tian, Kevin wrote:
> > > > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > Sent: Monday, May 20, 2024 10:19 AM
> > > >
> > > > On 5/15/24 4:50 PM, Tian, Kevin wrote:
> > > > > > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > > > Sent: Tuesday, April 30, 2024 10:57 PM
> > > > > >
> > > > > > @@ -308,6 +314,19 @@ int iommufd_hwpt_alloc(struct
> > iommufd_ucmd
> > > > > > *ucmd)
> > > > > >    		goto out_put_pt;
> > > > > >    	}
> > > > > >
> > > > > > +	if (cmd->flags & IOMMU_HWPT_FAULT_ID_VALID) {
> > > > > > +		struct iommufd_fault *fault;
> > > > > > +
> > > > > > +		fault = iommufd_get_fault(ucmd, cmd->fault_id);
> > > > > > +		if (IS_ERR(fault)) {
> > > > > > +			rc = PTR_ERR(fault);
> > > > > > +			goto out_hwpt;
> > > > > > +		}
> > > > > > +		hwpt->fault = fault;
> > > > > > +		hwpt->domain->iopf_handler = iommufd_fault_iopf_handler;
> > > > > > +		hwpt->domain->fault_data = hwpt;
> > > > > > +	}
> > > > >
> > > > > this is nesting specific. why not moving it to the nested_alloc()?
> > > >
> > > > Nesting is currently a use case for userspace I/O page faults, but this
> > > > design should be general enough to support other scenarios as well.
> > >
> > > Do we allow user page table w/o nesting?
> > >
> > > What would be a scenario in which the user doesn't manage the
> > > page table but still want to handle the I/O page fault? The fault
> > > should always be delivered to the owner managing the page table...
> >
> > userspace always manages the page table, either it updates the IOPTE
> > directly in a nest or it calls iommufd map operations.
> >
> > Ideally the driver will allow PRI on normal cases, although it will
> > probably never be used.
>
> But now it's done in a half way.
>
> valid_flags in normal cases doesn't accept a fault ID. but we then
> handle the fault ID flag generally above.
>
> I'd like to see a consistent message throughout the path.

Okay, I see. I think valid_flags logic is doing the right thing. It
indicates that user space page fault on a paging hwpt is not supported
yet, but it leaves the room to grow it in the future.

I will post v6 of this series soon to address some obvious issues
identified during this v5 review cycle. Thanks to all review comments.

Best regards,
baolu