[PATCH 04/27] drm/i915/gvt: Verify VFIO-pinned page is THP when shadowing 2M gtt entry

[Sean Christopherson]

When shadowing a GTT entry with a 2M page, explicitly verify that the
first page pinned by VFIO is a transparent hugepage instead of assuming
that page observed by is_2MB_gtt_possible() is the same page pinned by
vfio_pin_pages().  E.g. if userspace is doing something funky with the
guest's memslots, or if the page is demoted between is_2MB_gtt_possible()
and vfio_pin_pages().

This is more of a performance optimization than a bug fix as the check
for contiguous struct pages should guard against incorrect mapping (even
though assuming struct pages are virtually contiguous is wrong).

The real motivation for explicitly checking for a transparent hugepage
after pinning is that it will reduce the risk of introducing a bug in a
future fix for a page refcount leak (KVMGT doesn't put the reference
acquired by gfn_to_pfn()), and eventually will allow KVMGT to stop using
KVM's gfn_to_pfn() altogether.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 drivers/gpu/drm/i915/gvt/kvmgt.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index 714221f9a131..6f358b4fe406 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -159,11 +159,25 @@ static int gvt_pin_guest_page(struct intel_vgpu *vgpu, unsigned long gfn,
 			goto err;
 		}
 
-		if (npage == 0)
-			base_page = cur_page;
+		if (npage == 0) {
+			/*
+			 * Bail immediately to avoid unnecessary pinning when
+			 * trying to shadow a 2M page and the host page isn't
+			 * a transparent hugepage.
+			 *
+			 * TODO: support other type hugepages, e.g. HugeTLB.
+			 */
+			if (size == I915_GTT_PAGE_SIZE_2M &&
+			    !PageTransHuge(cur_page))
+				ret = -EIO;
+			else
+				base_page = cur_page;
+		}
 		else if (base_page + npage != cur_page) {
 			gvt_vgpu_err("The pages are not continuous\n");
 			ret = -EINVAL;
+		}
+		if (ret < 0) {
 			npage++;
 			goto err;
 		}
-- 
2.39.0.314.g84b9a713c41-goog