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

[Tang, CQ]

> -----Original Message-----
> From: Chris Wilson <chris at chris-wilson.co.uk>
> Sent: Tuesday, December 15, 2020 2:02 PM
> To: Tang, CQ <cq.tang at intel.com>; intel-gfx at lists.freedesktop.org
> Cc: stable@ <vger.kernel.org stable at vger.kernel.org>
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: Fix mismatch between misplaced
> vma check and vma insert
> 
> Quoting Tang, CQ (2020-12-15 21:50:53)
> >
> >
> > > -----Original Message-----
> > > From: Chris Wilson <chris at chris-wilson.co.uk>
> > > Sent: Tuesday, December 15, 2020 12:31 PM
> > > To: intel-gfx at lists.freedesktop.org
> > > Cc: Chris Wilson <chris at chris-wilson.co.uk>; Tang, CQ
> > > <cq.tang at intel.com>; stable at 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 at intel.com>
> > > Fixes: 48ea1e32c39d ("drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1
> > > page")
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: CQ Tang <cq.tang at intel.com>
> > > Cc: stable at 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

How do we trigger this code?  I run gem_exec_params at larger-than-life-batch but did not see this code is executed.
Basically how do we triggre first attempt to pin the object in place.

--CQ

> 
> -Chris