CONFIG_THUMB2_KERNEL=y boot failure after Spectre BHB fixes

[Nathan Chancellor <nathan@kernel.org>]

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Hi Russell,

Apologies if this has already been reported, I did not see anything when
sifting through lore.kernel.org and I can still reproduce this with
current mainline (1db333d9a51f).

I noticed a QEMU boot failure with multi_v7_defconfig with
CONFIG_THUMB2_KERNEL=y in our continuous integration [1]. It does not
appear to be compiler specific, as it reproduces with a bunch of
different clang versions and GCC 11.2.0 (I didn't try other GCC
versions).

At commit 04e91b732476 ("ARM: early traps initialisation"), everything
boots fine.

At commit 8d9d651ff227 ("ARM: use LOADADDR() to get load address of
sections"), there is no output from QEMU at all.

At commit b9baf5c8c5c3 ("ARM: Spectre-BHB workaround"), there is some
output but the boot still hangs before init. I have included a log of
the output of QEMU at this revision along with the command line I am
using, which comes from [2]. If I disable CONFIG_HARDEN_BRANCH_HISTORY,
the kernel boots.

If there is any further information I can provide or patches I can try,
I am happy to do so.

[1]: https://github.com/ClangBuiltLinux/continuous-integration2/runs/5496036256?check_suite_focus=true
[2]: https://github.com/ClangBuiltLinux/boot-utils

Cheers,
Nathan

[-- Attachment #2: boot.log --]
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+ timeout --foreground 1m stdbuf -oL -eL qemu-system-arm -initrd /home/nathan/cbl/github/boot-utils-ro/images/arm/rootfs.cpio -append 'console=ttyAMA0 earlycon' -machine virt -no-reboot -display none -kernel /home/nathan/cbl/src/linux/arch/arm/boot/zImage -m 512m -nodefaults -serial mon:stdio
[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 5.16.0-00004-gb9baf5c8c5c3 (nathan@dev-arch.thelio-3990X) (arm-none-eabi-gcc (Arch Repository) 11.2.0, GNU ld (GNU Binutils) 2.37) #1 SMP Thu Mar 10 12:03:57 MST 2022
[    0.000000] CPU: ARMv7 Processor [412fc0f1] revision 1 (ARMv7), cr=50c5387d
[    0.000000] CPU: div instructions available: patching division code
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
[    0.000000] OF: fdt: Machine model: linux,dummy-virt
[    0.000000] earlycon: pl11 at MMIO 0x09000000 (options '')
[    0.000000] printk: bootconsole [pl11] enabled
[    0.000000] Memory policy: Data cache writealloc
[    0.000000] efi: UEFI not found.
[    0.000000] cma: Reserved 64 MiB at 0x5c000000
[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000040000000-0x000000005fffffff]
[    0.000000]   Normal   empty
[    0.000000]   HighMem  empty
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000040000000-0x000000005fffffff]
[    0.000000] Initmem setup node 0 [mem 0x0000000040000000-0x000000005fffffff]
[    0.000000] psci: probing for conduit method from DT.
[    0.000000] psci: PSCIv1.1 detected in firmware.
[    0.000000] psci: Using standard PSCI v0.2 function IDs
[    0.000000] psci: Trusted OS migration not required
[    0.000000] psci: SMC Calling Convention v1.0
[    0.000000] percpu: Embedded 16 pages/cpu s33996 r8192 d23348 u65536
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 130048
[    0.000000] Kernel command line: console=ttyAMA0 earlycon
[    0.000000] Dentry cache hash table entries: 65536 (order: 6, 262144 bytes, linear)
[    0.000000] Inode-cache hash table entries: 32768 (order: 5, 131072 bytes, linear)
[    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
[    0.000000] Memory: 428336K/524288K available (10240K kernel code, 2268K rwdata, 5524K rodata, 2048K init, 392K bss, 30416K reserved, 65536K cma-reserved, 0K highmem)
[    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
[    0.000000] trace event string verifier disabled
[    0.000000] rcu: Hierarchical RCU implementation.
[    0.000000] rcu: 	RCU event tracing is enabled.
[    0.000000] rcu: 	RCU restricting CPUs from NR_CPUS=16 to nr_cpu_ids=1.
[    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 10 jiffies.
[    0.000000] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=1
[    0.000000] NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
[    0.000000] GICv2m: range[mem 0x08020000-0x08020fff], SPI[80:143]
[    0.000000] random: get_random_bytes called from _etext+0x600c95/0x67d464 with crng_init=0
[    0.000000] arch_timer: cp15 timer(s) running at 62.50MHz (virt).
[    0.000000] clocksource: arch_sys_counter: mask: 0x1ffffffffffffff max_cycles: 0x1cd42e208c, max_idle_ns: 881590405314 ns
[    0.000114] sched_clock: 57 bits at 62MHz, resolution 16ns, wraps every 4398046511096ns
[    0.000363] Switching to timer-based delay loop, resolution 16ns
[    0.002953] Console: colour dummy device 80x30
[    0.004065] Calibrating delay loop (skipped), value calculated using timer frequency.. 125.00 BogoMIPS (lpj=625000)
[    0.004331] pid_max: default: 32768 minimum: 301
[    0.005091] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
[    0.005239] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
qemu-system-arm: terminating on signal 15 from pid 511549 (timeout)
+ RET=124
+ set +x