[PATCH] i915: Drop relocation support on all new hardware (v3)
Zbigniew Kempczyński
zbigniew.kempczynski at intel.com
Thu Mar 11 18:19:53 UTC 2021
On Thu, Mar 11, 2021 at 11:18:11AM -0600, Jason Ekstrand wrote:
> On Thu, Mar 11, 2021 at 10:51 AM Zbigniew Kempczyński
> <zbigniew.kempczynski at intel.com> wrote:
> >
> > On Thu, Mar 11, 2021 at 10:24:38AM -0600, Jason Ekstrand wrote:
> > > On Thu, Mar 11, 2021 at 9:57 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> > > >
> > > > On Thu, Mar 11, 2021 at 4:50 PM Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > >
> > > > > On Thu, Mar 11, 2021 at 5:44 AM Zbigniew Kempczyński
> > > > > <zbigniew.kempczynski at intel.com> wrote:
> > > > > >
> > > > > > On Wed, Mar 10, 2021 at 03:50:07PM -0600, Jason Ekstrand wrote:
> > > > > > > The Vulkan driver in Mesa for Intel hardware never uses relocations if
> > > > > > > it's running on a version of i915 that supports at least softpin which
> > > > > > > all versions of i915 supporting Gen12 do. On the OpenGL side, Gen12+ is
> > > > > > > only supported by iris which never uses relocations. The older i965
> > > > > > > driver in Mesa does use relocations but it only supports Intel hardware
> > > > > > > through Gen11 and has been deprecated for all hardware Gen9+. The
> > > > > > > compute driver also never uses relocations. This only leaves the media
> > > > > > > driver which is supposed to be switching to softpin going forward.
> > > > > > > Making softpin a requirement for all future hardware seems reasonable.
> > > > > > >
> > > > > > > Rejecting relocations starting with Gen12 has the benefit that we don't
> > > > > > > have to bother supporting it on platforms with local memory. Given how
> > > > > > > much CPU touching of memory is required for relocations, not having to
> > > > > > > do so on platforms where not all memory is directly CPU-accessible
> > > > > > > carries significant advantages.
> > > > > > >
> > > > > > > v2 (Jason Ekstrand):
> > > > > > > - Allow TGL-LP platforms as they've already shipped
> > > > > > >
> > > > > > > v3 (Jason Ekstrand):
> > > > > > > - WARN_ON platforms with LMEM support in case the check is wrong
> > > > > >
> > > > > > I was asked to review of this patch. It works along with expected
> > > > > > IGT check https://patchwork.freedesktop.org/patch/423361/?series=82954&rev=25
> > > > > >
> > > > > > Before I'll give you r-b - isn't i915_gem_execbuffer2_ioctl() better place
> > > > > > to do for loop just after copy_from_user() and check relocation_count?
> > > > > > We have an access to exec2_list there, we know the gen so we're able to say
> > > > > > relocations are not supported immediate, without entering i915_gem_do_execbuffer().
> > > > >
> > > > > I considered that but it adds an extra object list walk for a case
> > > > > which we expect to not happen. I'm not sure how expensive the list
> > > > > walk would be if all we do is check the number of relocations on each
> > > > > object. I guess, if it comes right after a copy_from_user, it's all
> > > > > hot in the cache so it shouldn't matter. Ok. I've convinced myself.
> > > > > I'll move it.
> > > >
> > > > I really wouldn't move it if it's another list walk. Execbuf has a lot
> > > > of fast-paths going on, and we have extensive tests to make sure it
> > > > unwinds correctly in all cases. It's not very intuitive, but execbuf
> > > > code isn't scoring very high on that.
> > >
> > > And here I'd just finished doing the typing to move it. Good thing I
> > > hadn't closed vim yet and it was still in my undo buffer. :-)
> >
> > Before entering "slower" path from my perspective I would just check
> > batch object at that place. We still would have single list walkthrough
> > and quick check on the very beginning.
>
> Can you be more specific about what exactly you think we can check at
> the beginning? Either we add a flag that we can O(1) check at the
> beginning or we have to check everything in exec2_list for
> exec2_list[n].relocation_count == 0. That's a list walk. I'm not
> seeing what up-front check you're thinking we can do without the list
> walk.
I expect that last (default) or first (I915_EXEC_BATCH_FIRST) execobj
(batch) will likely has relocations. So we can check that single
object without entering i915_gem_do_execbuffer(). If that check
is missed (relocation_count = 0) you'll catch relocations in other
objects in check_relocations() as you already did. This is simple
optimization but we can avoid two iterations over buffer list
(first is in eb_lookup_vmas()).
--
Zbigniew
>
> --Jason
>
> > --
> > Zbigniew
> >
> > >
> > > --Jason
> > >
> > > > -Daniel
> > > >
> > > > >
> > > > > --Jason
> > > > >
> > > > > > --
> > > > > > Zbigniew
> > > > > >
> > > > > > >
> > > > > > > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > > > > > > Cc: Dave Airlie <airlied at redhat.com>
> > > > > > > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > > > > > > ---
> > > > > > > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 15 ++++++++++++---
> > > > > > > 1 file changed, 12 insertions(+), 3 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > > > > > > index 99772f37bff60..b02dbd16bfa03 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > > > > > > @@ -1764,7 +1764,8 @@ eb_relocate_vma_slow(struct i915_execbuffer *eb, struct eb_vma *ev)
> > > > > > > return err;
> > > > > > > }
> > > > > > >
> > > > > > > -static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
> > > > > > > +static int check_relocations(const struct i915_execbuffer *eb,
> > > > > > > + const struct drm_i915_gem_exec_object2 *entry)
> > > > > > > {
> > > > > > > const char __user *addr, *end;
> > > > > > > unsigned long size;
> > > > > > > @@ -1774,6 +1775,14 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
> > > > > > > if (size == 0)
> > > > > > > return 0;
> > > > > > >
> > > > > > > + /* Relocations are disallowed for all platforms after TGL-LP */
> > > > > > > + if (INTEL_GEN(eb->i915) >= 12 && !IS_TIGERLAKE(eb->i915))
> > > > > > > + return -EINVAL;
> > > > > > > +
> > > > > > > + /* All discrete memory platforms are Gen12 or above */
> > > > > > > + if (WARN_ON(HAS_LMEM(eb->i915)))
> > > > > > > + return -EINVAL;
> > > > > > > +
> > > > > > > if (size > N_RELOC(ULONG_MAX))
> > > > > > > return -EINVAL;
> > > > > > >
> > > > > > > @@ -1807,7 +1816,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
> > > > > > > if (nreloc == 0)
> > > > > > > continue;
> > > > > > >
> > > > > > > - err = check_relocations(&eb->exec[i]);
> > > > > > > + err = check_relocations(eb, &eb->exec[i]);
> > > > > > > if (err)
> > > > > > > goto err;
> > > > > > >
> > > > > > > @@ -1880,7 +1889,7 @@ static int eb_prefault_relocations(const struct i915_execbuffer *eb)
> > > > > > > for (i = 0; i < count; i++) {
> > > > > > > int err;
> > > > > > >
> > > > > > > - err = check_relocations(&eb->exec[i]);
> > > > > > > + err = check_relocations(eb, &eb->exec[i]);
> > > > > > > if (err)
> > > > > > > return err;
> > > > > > > }
> > > > > > > --
> > > > > > > 2.29.2
> > > > > > >
> > > > > > > _______________________________________________
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> > > > > _______________________________________________
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> > > >
> > > >
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch
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