[PATCH] i915/drm/gvt: initialize CSB tail value with zero

[Xinyun Liu]

When run `./drv_hangman --run-subtest hangcheck-unterminated` with
AcrnGT, vGPU reset falls into a dead loop because the original CSB tail
value (0xF) was not updated correctly. In fact, the value should be zero
after gpu reset caused by an invalid context. This dead loop also causes
the kernel panic if there is some graphics workload running on the vGPU.

 BUG: unable to handle kernel paging request at 00000000fffffffc
 IP: process_csb+0x14a/0x2a0
 PGD 0 P4D 0
 Oops: 0002 [#1] PREEMPT SMP
 Modules linked in: dwc3_pci dwc3 snd_usb_audio xhci_pci mei_me xhci_hcd snd_usbmidi_lib mei snd_hwdep hci_uart bluetooth ecdh_generic rfkill_gpio trusty_timer trusty_wall trusty_b
 CPU: 0 PID: 1371 Comm: kworker/0:1H Tainted: P     U  W  O    4.14.61-quilt-2e5dc0ac-g0feae7d57171 #2
 Hardware name:  ACRN-DM, BIOS 1.00 03/14/2014
 Workqueue: events_highpri i915_error_reset
 task: ffff88007cbc0040 task.stack: ffffc900010b0000
 RIP: 0010:process_csb+0x14a/0x2a0
 RSP: 0018:ffffc900010b3c90 EFLAGS: 00010206
 RAX: 00000000fffffffc RBX: ffffc90001e02370 RCX: 0000000000000008
 RDX: 0000000000000009 RSI: ffff88007c830308 RDI: 0000000000000000
 RBP: ffffc900010b3cd8 R08: 0000000000000001 R09: 0000000000002370
 R10: 0000000000000000 R11: 0000000000000000 R12: ffff88007c758000
 R13: 0000000000000007 R14: 0000000000000004 R15: ffff88007c830000
 FS:  0000000000000000(0000) GS:ffff88007f600000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000fffffffc CR3: 00000000796e0000 CR4: 00000000003406f0
 Call Trace:
  ? wake_up_process+0x20/0x20
  execlists_reset_prepare+0x65/0x120
  i915_gem_reset_prepare_engine+0x28/0x40
  i915_reset_engine+0x1e/0xe0
  i915_handle_error+0x117/0x470
  ? cpuacct_charge+0x81/0x90
  ? _raw_spin_unlock_irq+0x1e/0x40
  ? finish_task_switch+0x8d/0x1f0
  i915_error_reset+0x32/0x40
  process_one_work+0x186/0x3e0
  worker_thread+0x3d/0x3b0
  kthread+0x132/0x150
  ? process_one_work+0x3e0/0x3e0
  ? kthread_create_on_node+0x70/0x70
  ret_from_fork+0x3a/0x50
 Code: 00 00 44 89 00 48 83 c4 20 5b 41 5c 41 5d 41 5e 41 5f 5d c3 48 89 d8 31 d2 45 31 f6 e9 57 ff ff ff 0f 1f 44 00 00 48 85 c0 74 13 <f0> ff 08 0f 88 ed 6f 5d 00 75 08 48 89 c7
 RIP: process_csb+0x14a/0x2a0 RSP: ffffc900010b3c90
 CR2: 00000000fffffffc
 ---[ end trace 5751fb1d7b00b459 ]---

Link: https://lists.projectacrn.org/g/acrn-dev/message/11136
Signed-off-by: Xinyun Liu <xinyun.liu at intel.com>
---
 drivers/gpu/drm/i915/gvt/execlist.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gvt/execlist.c b/drivers/gpu/drm/i915/gvt/execlist.c
index 70494e394d2c..768e0b467a11 100644
--- a/drivers/gpu/drm/i915/gvt/execlist.c
+++ b/drivers/gpu/drm/i915/gvt/execlist.c
@@ -523,7 +523,7 @@ static void init_vgpu_execlist(struct intel_vgpu *vgpu, int ring_id)
 			_EL_OFFSET_STATUS_PTR);
 	ctx_status_ptr.dw = vgpu_vreg(vgpu, ctx_status_ptr_reg);
 	ctx_status_ptr.read_ptr = 0;
-	ctx_status_ptr.write_ptr = 0x7;
+	ctx_status_ptr.write_ptr = 0;
 	vgpu_vreg(vgpu, ctx_status_ptr_reg) = ctx_status_ptr.dw;
 }
 
-- 
2.18.0