Quoting Tvrtko Ursulin (2020-06-05 16:27:26)
>
> On 05/06/2020 10:58, Chris Wilson wrote:
> > Over the next couple of patches, we will want to lock all the modified
> > vma for relocation processing under a single ww_mutex. We neither want
> > to have to include the vma that are skipped (due to no modifications
> > required) nor do we want those to be marked as written too. So separate
> > out the reloc validation into an early step, which we can use both to
> > reject the execbuf before committing to making our changes, and to
> > filter out the unmodified vma.
> >
> > This does introduce a second pass through the reloc[], but only if we
> > need to emit relocations.
> >
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > ---
> > .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 178 +++++++++++++-----
> > 1 file changed, 133 insertions(+), 45 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > index cfe6d2cdbef1..7d4464fddca8 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -1331,6 +1331,117 @@ static u64
> > eb_relocate_entry(struct i915_execbuffer *eb,
> > struct eb_vma *ev,
> > const struct drm_i915_gem_relocation_entry *reloc)
> > +{
> > + struct eb_vma *target;
> > +
> > + /* we've already hold a reference to all valid objects */
> > + target = eb_get_vma(eb, reloc->target_handle);
> > + if (unlikely(!target))
> > + return -ENOENT;
> > +
> > + /*
> > + * If the relocation already has the right value in it, no
> > + * more work needs to be done.
> > + */
> > + if (gen8_canonical_addr(target->vma->node.start) == reloc->presumed_offset)
> > + return 0;
>
> These have been filtered out, no?
Only if the entire execobj->reloc[] was skipped. If some skipped and
some did not, we may end up here.
>
> > +
> > + /*
> > + * If we write into the object, we need to force the synchronisation
> > + * barrier, either with an asynchronous clflush or if we executed the
> > + * patching using the GPU (though that should be serialised by the
> > + * timeline). To be completely sure, and since we are required to
> > + * do relocations we are already stalling, disable the user's opt
> > + * out of our synchronisation.
> > + */
> > + ev->flags &= ~EXEC_OBJECT_ASYNC;
> > +
> > + /* and update the user's relocation entry */
> > + return relocate_entry(eb, ev->vma, reloc, target->vma);
> > +}
> > +
> > +static int eb_relocate_vma(struct i915_execbuffer *eb, struct eb_vma *ev)
> > +{
> > +#define N_RELOC(x) ((x) / sizeof(struct drm_i915_gem_relocation_entry))
> > + struct drm_i915_gem_relocation_entry stack[N_RELOC(512)];
> > + const struct drm_i915_gem_exec_object2 *entry = ev->exec;
> > + struct drm_i915_gem_relocation_entry __user *urelocs =
> > + u64_to_user_ptr(entry->relocs_ptr);
> > + unsigned long remain = entry->relocation_count;
> > +
> > + if (unlikely(remain > N_RELOC(ULONG_MAX)))
> > + return -EINVAL;
>
> This has been checked already in eb_reloca_vma_validate.
Ok. It didn't even register.
>
> > +
> > + /*
> > + * We must check that the entire relocation array is safe
> > + * to read. However, if the array is not writable the user loses
> > + * the updated relocation values.
> > + */
> > + if (unlikely(!access_ok(urelocs, remain * sizeof(*urelocs))))
> > + return -EFAULT;
> > +
> > + do {
> > + struct drm_i915_gem_relocation_entry *r = stack;
> > + unsigned int count =
> > + min_t(unsigned long, remain, ARRAY_SIZE(stack));
> > + unsigned int copied;
> > +
> > + /*
> > + * This is the fast path and we cannot handle a pagefault
> > + * whilst holding the struct mutex lest the user pass in the
> > + * relocations contained within a mmaped bo. For in such a case
> > + * we, the page fault handler would call i915_gem_fault() and
> > + * we would try to acquire the struct mutex again. Obviously
> > + * this is bad and so lockdep complains vehemently.
> > + */
> > + copied = __copy_from_user(r, urelocs, count * sizeof(r[0]));
> > + if (unlikely(copied))
> > + return -EFAULT;
> > +
> > + remain -= count;
>
> The above two comments end up duplicated which is kind of ugly. Not sure
> how a common "runner/looper" would look with just the per-reloc body
> being a passed in function.
I looked and thought it would just be the outer pair of loops being saved.
Still probably worth it, but it felt like more work than cut'n'paste!
>
> > + do {
> > + u64 offset = eb_relocate_entry(eb, ev, r);
> > +
> > + if (likely(offset == 0)) {
> > + } else if ((s64)offset < 0) {
> > + return (int)offset;
> > + } else {
> > + /*
> > + * Note that reporting an error now
> > + * leaves everything in an inconsistent
> > + * state as we have *already* changed
> > + * the relocation value inside the
> > + * object. As we have not changed the
> > + * reloc.presumed_offset or will not
> > + * change the execobject.offset, on the
> > + * call we may not rewrite the value
> > + * inside the object, leaving it
> > + * dangling and causing a GPU hang. Unless
> > + * userspace dynamically rebuilds the
> > + * relocations on each execbuf rather than
> > + * presume a static tree.
> > + *
> > + * We did previously check if the relocations
> > + * were writable (access_ok), an error now
> > + * would be a strange race with mprotect,
> > + * having already demonstrated that we
> > + * can read from this userspace address.
> > + */
> > + offset = gen8_canonical_addr(offset & ~UPDATE);
> > + __put_user(offset,
> > + &urelocs[r - stack].presumed_offset);
> > + }
> > + } while (r++, --count);
> > + urelocs += ARRAY_SIZE(stack);
> > + } while (remain);
> > +
> > + return 0;
> > +}
> > +
> > +static int
> > +eb_reloc_valid(struct i915_execbuffer *eb,
> > + struct eb_vma *ev,
> > + const struct drm_i915_gem_relocation_entry *reloc)
>
> It does a bit more than check for validity so if you agree maybe
> eb_reloc_prepare(_entry)?
You mean the deeply buried gen6 w/a
eb_reloc_prepare doesn't sound too bad.
-Chris
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