Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

[Jason Gunthorpe]

On Wed, Nov 08, 2023 at 08:53:00AM +0000, Tian, Kevin wrote:

> > There are many events related to object in guest memory or controlled
> > by the guest, eg C_BAD_CD and C_BAD_STE. These should be relayed or
> > the emulation is not working well.
> 
> so that's the category of unrecoverable faults?

I haven't looked exhaustively but I do have the impression that the
only recoverable fault is the 'page not present' one.

> btw I can understand C_BAD_CD given it's walked by the physical SMMU
> in nested configuration. But presumably STE is created by the smmu
> driver itself then why would there be an error to be relayed for
> guest STE?

If the guest programs a bad STE it should still generate a C_BAD_STE
even if the mediation SW could theoretically sanitize it (but sanitize
it to what? BLOCKED?). Since we have to forward things like C_BAD_CD
and others we may as well just drop an invalid STE and forward the
event like real HW.

> > > but I didn't get the last piece. If those domains are created by kernel
> > > drivers why would they require a uAPI for userspace to specify fault
> > > capable?
> > 
> > Not to userspace, but a kapi to request a fault capable domain and to
> > supply the fault handler. Eg:
> > 
> >  iommu_domain_alloc_faultable(dev, handler);
> 
> Does it affect SVA too?

Inside the driver the SVA should be constructed out of the same fault
handling infrastructure, but a SVA domain allocation should have a
different allocation function.

Jason