[Intel-gfx] [PATCH 4/4] i915: fix remap_io_sg to verify the pgprot

[Christoph Hellwig]

On Mon, May 17, 2021 at 06:06:44PM +0100, Matthew Auld wrote:
> > Looks like it is caused by the validation failure then.  Which means the
> > existing code is doing something wrong in its choice of the page
> > protection bit.  I really need help from the i915 maintainers here..
> 
> AFAIK there are two users of remap_io_sg, the first is our shmem
> objects(see i915_gem_shmem.c), and for these we support UC, WC, and WB
> mmap modes for userspace. The other user is device local-memory
> objects(VRAM), and for this one we have an actual io_mapping which is
> allocated as WC, and IIRC this should only be mapped as WC for the
> mmap mode, but normal userspace can't hit this path yet.

The only caller in current mainline is vm_fault_cpu in i915_gem_mman.c.
Is that device local?

> What do we need to do here? It sounds like shmem backed objects are
> allocated as WB for the pages underneath, but i915 allows mapping them
> as UC/WC which trips up this track_pfn thing?

To me the warnings looks like system memory is mapped with the wrong
permissions, yes.  If you want to map it as UC/WC the right set_memory_*
needs to be used on the kernel mapping as well to ensure that the
attributes don't conflict.