xen/arm: Properly map the FDT in the boot page table

xen/arm: Properly map the FDT in the boot page table

Currently, Xen is assuming the FDT will always fit in a 2MB section.
Recently, I noticed an early crash on Xen when using GRUB with the
following call trace:

(XEN) Hypervisor Trap. HSR=0x96000006 EC=0x25 IL=1 Syndrome=0x6
(XEN) CPU0: Unexpected Trap: Hypervisor
(XEN) ----[ Xen-4.9-unstable  arm64  debug=y   Not tainted ]----
(XEN) CPU:    0
(XEN) PC:     0000000000264140 strlen+0x10/0x84
(XEN) LR:     00000000002401c0
(XEN) SP:     00000000002cfc20
(XEN) CPSR:   400003c9 MODE:64-bit EL2h (Hypervisor, handler)
(XEN)      X0: 0000000000801230  X1: 0000000000801230  X2: 0000000000005230
(XEN)      X3: 0000000000000030  X4: 0000000000000030  X5: 0000000000000038
(XEN)      X6: 0000000000000034  X7: 0000000000000000  X8: 7f7f7f7f7f7f7f7f
(XEN)      X9: 64622c6479687222 X10: 7f7f7f7f7f7f7f7f X11: 0101010101010101
(XEN)     X12: 0000000000000030 X13: ffffff00ff000000 X14: 0800000003000000
(XEN)     X15: ffffffffffffffff X16: 00000000fefff610 X17: 00000000000000f0
(XEN)     X18: 0000000000000004 X19: 0000000000000008 X20: 00000000007fc040
(XEN)     X21: 00000000007fc000 X22: 000000000000000e X23: 0000000000000000
(XEN)     X24: 00000000002a9f58 X25: 0000000000801230 X26: 00000000002a9f68
(XEN)     X27: 00000000002a9f58 X28: 0000000000298910  FP: 00000000002cfc20
(XEN)
(XEN)   VTCR_EL2: 80010c40
(XEN)  VTTBR_EL2: 0000082800203000
(XEN)
(XEN)  SCTLR_EL2: 30c5183d
(XEN)    HCR_EL2: 000000000038663f
(XEN)  TTBR0_EL2: 00000000f4912000
(XEN)
(XEN)    ESR_EL2: 96000006
(XEN)  HPFAR_EL2: 00000000e8071000
(XEN)    FAR_EL2: 0000000000801230
(XEN)
(XEN) Xen stack trace from sp=00000000002cfc20:
(XEN)    00000000002cfc70 0000000000240254 00000000002a9f58 00000000007fc000
(XEN)    0000000000000000 0000000000000000 0000000000000000 00000000007fc03c
(XEN)    00000000002cfd78 0000000000000000 00000000002cfca0 00000000002986fc
(XEN)    0000000000000000 00000000007fc000 0000000000000000 0000000000000000
(XEN)    00000000002cfcc0 0000000000298f1c 0000000000000000 00000000007fc000
(XEN)    00000000002cfdc0 000000000029904c 00000000f47fc000 00000000f4604000
(XEN)    00000000f47fc000 00000000007fc000 0000000000400000 0000000000000100
(XEN)    00000000f4604000 0000000000000001 0000000000000001 8000000000000002
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 00000000002cfdc0 0000000000299038
(XEN)    00000000f47fc000 00000000f4604000 00000000f47fc000 0000000000000000
(XEN)    00000000002cfe20 000000000029c420 00000000002d8000 00000000f4604000
(XEN)    00000000f47fc000 0000000000000000 0000000000400000 0000000000000100
(XEN)    00000000f4604000 0000000000000001 00000000f47fc000 000000000029c404
(XEN)    00000000fefff510 0000000000200624 00000000f4804000 00000000f4604000
(XEN)    00000000f47fc000 0000000000000000 0000000000400000 0000000000000100
(XEN)    0000000000000001 0000000000000001 0000000000000001 8000000000000002
(XEN)    00000000f47fc000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN) Xen call trace:
(XEN)    [<0000000000264140>] strlen+0x10/0x84 (PC)
(XEN)    [<00000000002401c0>] fdt_get_property_namelen+0x9c/0xf0 (LR)
(XEN)    [<0000000000240254>] fdt_get_property+0x40/0x50
(XEN)    [<00000000002986fc>] bootfdt.c#device_tree_get_u32+0x18/0x5c
(XEN)    [<0000000000298f1c>] device_tree_for_each_node+0x84/0x144
(XEN)    [<000000000029904c>] boot_fdt_info+0x70/0x23c
(XEN)    [<000000000029c420>] start_xen+0x9c/0xd30
(XEN)    [<0000000000200624>] arm64/head.o#paging+0x84/0xbc
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) CPU0: Unexpected Trap: Hypervisor
(XEN)
(XEN) ****************************************

Indeed, the booting documentation for AArch32 and AArch64 only requires
the FDT to be placed on a 8-byte boundary. This means the Device-Tree can
cross a 2MB boundary.

Given that Xen limits the size of the FDT to 2MB, it will always fit in
a 4MB slot. So extend the fixmap slot for FDT from 2MB to 4MB.

The second 2MB superpage will only be mapped if the FDT is cross the 2MB
boundary.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>