drm/i915/kvmgt: fix suspicious rcu dereference usage

The srcu read lock must be held while accessing kvm memslots.
This patch fix below warning for function kvmgt_rw_gpa().

[  165.345093] [ ERR: suspicious RCU usage.  ]
[  165.416538] Call Trace:
[  165.418989]  dump_stack+0x85/0xc2
[  165.422310]  lockdep_rcu_suspicious+0xd7/0x110
[  165.426769]  kvm_read_guest_page+0x195/0x1b0 [kvm]
[  165.431574]  kvm_read_guest+0x50/0x90 [kvm]
[  165.440492]  kvmgt_rw_gpa+0x43/0xa0 [kvmgt]
[  165.444683]  kvmgt_read_gpa+0x11/0x20 [kvmgt]
[  165.449061]  gtt_get_entry64+0x4d/0xc0 [i915]
[  165.453438]  ppgtt_populate_shadow_page_by_guest_entry+0x380/0xdc0 [i915]
[  165.460254]  shadow_mm+0xd1/0x460 [i915]
[  165.472488]  intel_vgpu_create_mm+0x1ab/0x210 [i915]
[  165.477472]  intel_vgpu_g2v_create_ppgtt_mm+0x5f/0xc0 [i915]
[  165.483154]  pvinfo_mmio_write+0x19b/0x1d0 [i915]
[  165.499068]  intel_vgpu_emulate_mmio_write+0x3f9/0x600 [i915]
[  165.504827]  intel_vgpu_rw+0x114/0x150 [kvmgt]
[  165.509281]  intel_vgpu_write+0x16f/0x1a0 [kvmgt]
[  165.513993]  vfio_mdev_write+0x20/0x30 [vfio_mdev]
[  165.518793]  vfio_device_fops_write+0x24/0x30 [vfio]
[  165.523770]  __vfs_write+0x28/0x120
[  165.540529]  vfs_write+0xce/0x1f0

v2: fix Cc format for stable

Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: <stable@vger.kernel.org> # v4.10+
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -1424,7 +1424,7 @@ static int kvmgt_rw_gpa(unsigned long handle, unsigned long gpa,
 {
 	struct kvmgt_guest_info *info;
 	struct kvm *kvm;
-	int ret;
+	int idx, ret;
 	bool kthread = current->mm == NULL;
 
 	if (!handle_valid(handle))
@@ -1436,8 +1436,10 @@ static int kvmgt_rw_gpa(unsigned long handle, unsigned long gpa,
 	if (kthread)
 		use_mm(kvm->mm);
 
+	idx = srcu_read_lock(&kvm->srcu);
 	ret = write ? kvm_write_guest(kvm, gpa, buf, len) :
 		      kvm_read_guest(kvm, gpa, buf, len);
+	srcu_read_unlock(&kvm->srcu, idx);
 
 	if (kthread)
 		unuse_mm(kvm->mm);