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

[Matthew Auld]

On Mon, 17 May 2021 at 14:11, Christoph Hellwig <hch at lst.de> wrote:
>
> On Mon, May 17, 2021 at 04:09:42PM +0300, Serge Belyshev wrote:
> > Christoph Hellwig <hch at lst.de> writes:
> >
> > > As an ad-hoc experiment:  can you replace the call to remap_pfn_range
> > > with remap_pfn_range_notrack (and export it if you build i915 modular)
> > > in remap_io_sg and see if that makes any difference?
> >
> > That worked, thanks -- no artifacts seen.
>
> 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.

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?

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