Re: [PATCH RFC 08/12] iommufd: IOCTLs for the io_pagetable

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Mar 23, 2022 at 01:10:38PM -0600, Alex Williamson wrote:
> On Fri, 18 Mar 2022 14:27:33 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > +static int conv_iommu_prot(u32 map_flags)
> > +{
> > +	int iommu_prot;
> > +
> > +	/*
> > +	 * We provide no manual cache coherency ioctls to userspace and most
> > +	 * architectures make the CPU ops for cache flushing privileged.
> > +	 * Therefore we require the underlying IOMMU to support CPU coherent
> > +	 * operation.
> > +	 */
> > +	iommu_prot = IOMMU_CACHE;
> 
> Where is this requirement enforced?  AIUI we'd need to test
> IOMMU_CAP_CACHE_COHERENCY somewhere since functions like
> intel_iommu_map() simply drop the flag when not supported by HW.

You are right, the correct thing to do is to fail device
binding/attach entirely if IOMMU_CAP_CACHE_COHERENCY is not there,
however we can't do that because Intel abuses the meaning of
IOMMU_CAP_CACHE_COHERENCY to mean their special no-snoop behavior is
supported.

I want Intel to split out their special no-snoop from IOMMU_CACHE and
IOMMU_CAP_CACHE_COHERENCY so these things have a consisent meaning in
all iommu drivers. Once this is done vfio and iommufd should both
always set IOMMU_CACHE and refuse to work without
IOMMU_CAP_CACHE_COHERENCY. (unless someone knows of an !IOMMU_CACHE
arch that does in fact work today with vfio, somehow, but I don't..)

I added a fixme about this.

> This also seems like an issue relative to vfio compatibility that I
> don't see mentioned in that patch.  Thanks,

Yes, it was missed in the notes for vfio compat that Intel no-snoop is
not working currently, I fixed it.

Thanks,
Jason