[Intel-gfx] [PATCH 5.4.y] drm/i915: Don't use BAR mappings for ring buffers with LLC

[Jason Andryuk]

On Fri, Mar 17, 2023 at 8:58 AM Greg KH <gregkh at linuxfoundation.org> wrote:
>
> On Thu, Mar 16, 2023 at 01:58:35PM -0700, John Harrison wrote:
> > On 3/15/2023 10:57, Greg KH wrote:
> > > On Wed, Mar 15, 2023 at 10:07:53AM -0700, John Harrison wrote:
> > > > On 3/15/2023 00:51, Greg KH wrote:
> > > > > On Mon, Mar 13, 2023 at 07:22:11PM -0700, John.C.Harrison at Intel.com wrote:
> > > > > > From: John Harrison <John.C.Harrison at Intel.com>
> > > > > >
> > > > > > Direction from hardware is that ring buffers should never be mapped
> > > > > > via the BAR on systems with LLC. There are too many caching pitfalls
> > > > > > due to the way BAR accesses are routed. So it is safest to just not
> > > > > > use it.
> > > > > >
> > > > > > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> > > > > > Fixes: 9d80841ea4c9 ("drm/i915: Allow ringbuffers to be bound anywhere")
> > > > > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > > > > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > > > > > Cc: Jani Nikula <jani.nikula at linux.intel.com>
> > > > > > Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > > > > > Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> > > > > > Cc: intel-gfx at lists.freedesktop.org
> > > > > > Cc: <stable at vger.kernel.org> # v4.9+
> > > > > > Tested-by: Jouni Högander <jouni.hogander at intel.com>
> > > > > > Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > > > > > Link: https://patchwork.freedesktop.org/patch/msgid/20230216011101.1909009-3-John.C.Harrison@Intel.com
> > > > > > (cherry picked from commit 65c08339db1ada87afd6cfe7db8e60bb4851d919)
> > > > > > Signed-off-by: Jani Nikula <jani.nikula at intel.com>
> > > > > > (cherry picked from commit 85636167e3206c3fbd52254fc432991cc4e90194)
> > > > > > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> > > > > > ---
> > > > > >    drivers/gpu/drm/i915/gt/intel_ringbuffer.c | 4 ++--
> > > > > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > Also queued up for 5.10.y, you forgot that one :)
> > > > I'm still working through the backlog of them.
> > > >
> > > > Note that these patches must all be applied as a pair. The 'don't use
> > > > stolen' can be applied in isolation but won't totally fix the problem.
> > > > However, applying 'don't use BAR mappings' without applying the stolen patch
> > > > first will results in problems such as the failure to boot that was recently
> > > > reported and resulted in a revert in one of the trees.
> > > I do not understand, you only submitted 1 patch here, what is the
> > > "pair"?
> > The original patch series was two patches -
> > https://patchwork.freedesktop.org/series/114080/. One to not use stolen
> > memory and the other to not use BAR mappings. If the anti-BAR patch is
> > applied without the anti-stolen patch then the i915 driver will attempt to
> > access stolen memory directly which will fail. So both patches must be
> > applied and in the correct order to fix the problem of cache aliasing when
> > using BAR accesses on LLC systems.
> >
> > As above, I am working my way through the bunch of 'FAILED patch' emails.
> > The what-to-do instructions in those emails explicitly say to send the patch
> > individually in reply to the 'FAILED' message rather than as part of any
> > original series.
>
> So what commits exactly in Linus's tree should be in these stable
> branches?  Sorry, I still do not understand if we are missing one or if
> we need to revert something.

Hi, Greg,

5.4.237 fails to boot as a Xen PV Dom0.  It hangs after modprobe
i915.ko with no further output, though it seems to response to magic
sysrq.

Reverting 5.4.y commit 1aed78cfda7f17f3cc71cb127a85a188eafc679a
("drm/i915: Don't use BAR mappings for ring buffers with LLC") lets it
boot properly.

Thanks,
Jason