If GUP-ineligible pages are passed to a GEM userptr object, -EFAULT is
returned only when the object is actually bound.

The xf86-video-intel userspace driver cannot differentiate this
condition, and marks the GPU as wedged.  This disables graphics
acceleration and may cripple other functionalities such as VT switch.

Solve this by "prefaulting" user pages on GEM object creation, testing
whether all pages are eligible for get_user_pages() in the process.
On failure, return -EFAULT so that userspace can fallback to software
bliting.

This behavior can be controlled by using a new modparam
"gem_userptr_prefault", which is true by default.

The only known use for this patch is Qubes OS's GUI virtualization.
Userspace is expected to resort to DMA-BUF whenever possible.

Signed-off-by: Jinoh Kang <jinoh.kang...@gmail.com>
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 36 +++++++++++++++++++++++++
 drivers/gpu/drm/i915/i915_params.c      |  3 +++
 drivers/gpu/drm/i915/i915_params.h      |  1 +
 3 files changed, 40 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c 
b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 961abb6ea18e..c1e680cf037d 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -748,6 +748,34 @@ static const struct drm_i915_gem_object_ops 
i915_gem_userptr_ops = {
        .release = i915_gem_userptr_release,
 };

+static int i915_gem_userptr_prefault(unsigned long start,
+                                    unsigned long nr_pages,
+                                    bool readonly)
+{
+       struct mm_struct *mm = current->mm;
+       unsigned int gup_flags = (readonly ? 0 : FOLL_WRITE) | FOLL_NOWAIT;
+       int err = 0;
+
+       down_read(&mm->mmap_sem);
+       while (nr_pages) {
+               long ret;
+
+               ret = get_user_pages(start, nr_pages, gup_flags, NULL, NULL);
+               if (ret < 0) {
+                       err = (int)ret;
+                       break;
+               }
+               if (ret == 0)
+                       ret = 1;  /* skip this page */
+
+               start += ret << PAGE_SHIFT;
+               nr_pages -= ret;
+       }
+       up_read(&mm->mmap_sem);
+
+       return err;
+}
+
 /*
  * Creates a new mm object that wraps some normal memory from the process
  * context - user memory.
@@ -815,6 +843,14 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
                       (char __user *)(unsigned long)args->user_ptr, 
args->user_size))
                return -EFAULT;

+       if (READ_ONCE(i915_modparams.gem_userptr_prefault)) {
+               ret = i915_gem_userptr_prefault((unsigned long)args->user_ptr,
+                                               args->user_size >> PAGE_SHIFT,
+                                               args->flags & 
I915_USERPTR_READ_ONLY);
+               if (ret)
+                       return ret;
+       }
+
        if (args->flags & I915_USERPTR_READ_ONLY) {
                struct i915_hw_ppgtt *ppgtt;

diff --git a/drivers/gpu/drm/i915/i915_params.c 
b/drivers/gpu/drm/i915/i915_params.c
index 295e981e4a39..702163d85921 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -174,6 +174,9 @@ i915_param_named(enable_dpcd_backlight, bool, 0600,
 i915_param_named(enable_gvt, bool, 0400,
        "Enable support for Intel GVT-g graphics virtualization host 
support(default:false)");

+i915_param_named(gem_userptr_prefault, bool, 0600,
+       "Prefault pages when userptr GEM object is created (default:true)");
+
 static __always_inline void _print_param(struct drm_printer *p,
                                         const char *name,
                                         const char *type,
diff --git a/drivers/gpu/drm/i915/i915_params.h 
b/drivers/gpu/drm/i915/i915_params.h
index 6c4d4a21474b..86915d1fc303 100644
--- a/drivers/gpu/drm/i915/i915_params.h
+++ b/drivers/gpu/drm/i915/i915_params.h
@@ -66,6 +66,7 @@ struct drm_printer;
        param(bool, disable_display, false) \
        param(bool, verbose_state_checks, true) \
        param(bool, nuclear_pageflip, false) \
+       param(bool, gem_userptr_prefault, true) \
        param(bool, enable_dp_mst, true) \
        param(bool, enable_dpcd_backlight, false) \
        param(bool, enable_gvt, false)
-- 
2.26.2


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