This can fix below oops. The target pfn must be mem backed.

[ 3639.109674] BUG: unable to handle kernel paging request at ffff8c44832a3000
[ 3639.109681] IP: memcpy_erms+0x6/0x10
[ 3639.109682] PGD 0 P4D 0
[ 3639.109685] Oops: 0000 1 SMP PTI
[ 3639.109726] CPU: 2 PID: 1724 Comm: qemu-system-x86 Not tainted 4.16.0-rc5+ #1
[ 3639.109727] Hardware name: /NUC7i7BNB, BIOS BNKBL357.86A.0050.2017.0816.2002 08/16/2017
[ 3639.109729] RIP: 0010:memcpy_erms+0x6/0x10
[ 3639.109730] RSP: 0018:ffffb1b7c3fbbbf0 EFLAGS: 00010246
[ 3639.109731] RAX: ffff8a44b6460000 RBX: 0000000036460000 RCX: 0000000000001000
[ 3639.109732] RDX: 0000000000001000 RSI: ffff8c44832a3000 RDI: ffff8a44b6460000
[ 3639.109733] RBP: 000000000006c8c0 R08: ffff8a44b6460000 R09: 0000000000000000
[ 3639.109734] R10: ffffb1b7c3fbbcd0 R11: ffff8a4d102018c0 R12: 0000000000000000
[ 3639.109734] R13: 0000000000000002 R14: 0000000000200000 R15: 0000000000000000
[ 3639.109736] FS: 00007f37f6d09700(0000) GS:ffff8a4d36d00000(0000) knlGS:0000000000000000
[ 3639.109737] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3639.109738] CR2: ffff8c44832a3000 CR3: 000000088b7b8004 CR4: 00000000003626e0
[ 3639.109739] Call Trace:
[ 3639.109743] swiotlb_tbl_map_single+0x2bb/0x300
[ 3639.109746] map_single+0x30/0x80
[ 3639.109748] swiotlb_map_page+0x87/0x150
[ 3639.109751] kvmgt_dma_map_guest_page+0x329/0x3a0 [kvmgt]
[ 3639.109764] ? kvm_write_guest_offset_cached+0x84/0xe0 [kvm]
[ 3639.109789] intel_vgpu_emulate_ggtt_mmio_write+0x1f4/0x250 [i915]
[ 3639.109808] intel_vgpu_emulate_mmio_write+0x162/0x230 [i915]
[ 3639.109811] intel_vgpu_rw+0x1fc/0x240 [kvmgt]
[ 3639.109813] intel_vgpu_write+0x164/0x1f0 [kvmgt]
[ 3639.109816] __vfs_write+0x33/0x170
[ 3639.109818] ? do_vfs_ioctl+0x9f/0x5f0
[ 3639.109820] vfs_write+0xb3/0x1a0
[ 3639.109822] SyS_pwrite64+0x90/0xb0
[ 3639.109825] do_syscall_64+0x68/0x120
[ 3639.109827] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 3639.109829] RIP: 0033:0x7f3802b2d873
[ 3639.109830] RSP: 002b:00007f37f6d08670 EFLAGS: 00000293 ORIG_RAX: 0000000000000012
[ 3639.109831] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f3802b2d873
[ 3639.109832] RDX: 0000000000000008 RSI: 00007f37f6d086a0 RDI: 000000000000001a
[ 3639.109833] RBP: 00007f37f6d086c0 R08: 0000000000000008 R09: ffffffffffffffff
[ 3639.109834] R10: 00000000008041c8 R11: 0000000000000293 R12: 00007ffd8bbf92ae
[ 3639.109835] R13: 00007ffd8bbf92af R14: 00007f37f6d09700 R15: 00007f37f6d099c0

v2: add Fixes tag.
Signed-off-by: Changbin Du <changbin.du@intel.com>
Fixes: cf4ee73f ("drm/i915/gvt: Fix guest vGPU hang caused by very high dma setup overhead")
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>