Re: [Intel-gfx] [PATCH] drm/i915: Fix mismatch between misplaced vma check and vma insert

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tang, CQ (2020-12-15 21:50:53)
> 
> 
> > -----Original Message-----
> > From: Chris Wilson <chris@chris-wilson.co.uk>
> > Sent: Tuesday, December 15, 2020 12:31 PM
> > To: intel-gfx@lists.freedesktop.org
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>; Tang, CQ <cq.tang@intel.com>;
> > stable@vger.kernel.org
> > Subject: [PATCH] drm/i915: Fix mismatch between misplaced vma check and
> > vma insert
> > 
> > When inserting a VMA, we restrict the placement to the low 4G unless the
> > caller opts into using the full range. This was done to allow usersapce the
> > opportunity to transition slowly from a 32b address space, and to avoid
> > breaking inherent 32b assumptions of some commands.
> > 
> > However, for insert we limited ourselves to 4G-4K, but on verification we
> > allowed the full 4G. This causes some attempts to bind a new buffer to
> > sporadically fail with -ENOSPC, but at other times be bound successfully.
> > 
> > commit 48ea1e32c39d ("drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1
> > page") suggests that there is a genuine problem with stateless addressing
> > that cannot utilize the last page in 4G and so we purposefully excluded it.
> > 
> > Reported-by: CQ Tang <cq.tang@intel.com>
> > Fixes: 48ea1e32c39d ("drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1
> > page")
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: CQ Tang <cq.tang@intel.com>
> > Cc: stable@vger.kernel.org
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > index 193996144c84..2ff32daa50bd 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -382,7 +382,7 @@ eb_vma_misplaced(const struct
> > drm_i915_gem_exec_object2 *entry,
> >               return true;
> > 
> >       if (!(flags & EXEC_OBJECT_SUPPORTS_48B_ADDRESS) &&
> > -         (vma->node.start + vma->node.size - 1) >> 32)
> > +         (vma->node.start + vma->node.size + 4095) >> 32)
> 
> Why 4095 not 4096?

It's the nature of the test that we need an inclusive bound.

Consider an object of size 4G - 4K, that is allowed to fit within our 32b
GTT.

	4G - 4k + 4K = 4G == 1 << 32: => vma misplaced

	4G - 4k + 4k - 1 = 4G -1 = 0xffffffff => vma ok

-Chris
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