[PATCH 2/2] drm/i915/gvt: A workaround for an regression caused by one i915 PPGTT optimization

[Zhi Wang]

In the commit:

commit 14826673247eaf36b16fd821fac27efa663f3fa6
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Fri Sep 8 19:16:22 2017 +0100

    drm/i915: Only initialize partially filled pagetables

    If we know that we will completely fill a pagetable (i.e. we are
    inserting a complete set of 512 pages), we can skip prefilling that PT
    with scratch entries. If we have to abort the insertion prior to writing
    the real entries, we will teardown the pagetable and remove it from the
    page directory (so that we will restart the allocation next time).

    We could do similar tricks for the PD and PDP, but the likelihood of a
    single insertion covering the entire 512 entries diminishes, as do the
    cycle savings. The saving are even greater (relatively) when we are
    preallocating page tables for huge pages, as then we never need to fill
    the page table.

It will link an un-initialized PTE page into a PPGTT page table tracked by
GVT-g, which leads to linux guest failing to boot. Since the fix of root
casue still needs some time to be ready, a temporary workaround is
introduced first.

Signed-off-by: Zhi Wang <zhi.a.wang at intel.com>
---
 drivers/gpu/drm/i915/gvt/gtt.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/gtt.c b/drivers/gpu/drm/i915/gvt/gtt.c
index 6fa9271..d24d52d 100644
--- a/drivers/gpu/drm/i915/gvt/gtt.c
+++ b/drivers/gpu/drm/i915/gvt/gtt.c
@@ -767,7 +767,6 @@ static int ppgtt_write_protection_handler(void *data, u64 pa,
 {
 	struct intel_vgpu_page_track *t = data;
 	struct intel_vgpu_guest_page *p = page_track_to_guest_page(t);
-	int ret;
 
 	if (bytes != 4 && bytes != 8)
 		return -EINVAL;
@@ -775,11 +774,9 @@ static int ppgtt_write_protection_handler(void *data, u64 pa,
 	if (!t->tracked)
 		return -EINVAL;
 
-	ret = ppgtt_handle_guest_write_page_table_bytes(p,
+	ppgtt_handle_guest_write_page_table_bytes(p,
 		pa, p_data, bytes);
-	if (ret)
-		return ret;
-	return ret;
+	return 0;
 }
 
 static int reclaim_one_mm(struct intel_gvt *gvt);
-- 
2.7.4