Re: [PATCHv2] arm64: initialize per-cpu offsets earlier

[Qian Cai]

On Mon, 2020-10-05 at 17:43 +0100, Mark Rutland wrote:
> The current initialization of the per-cpu offset register is difficult
> to follow and this initialization is not always early enough for
> upcoming instrumentation with KCSAN, where the instrumentation callbacks
> use the per-cpu offset.
> 
> To make it possible to support KCSAN, and to simplify reasoning about
> early bringup code, let's initialize the per-cpu offset earlier, before
> we run any C code that may consume it. To do so, this patch adds a new
> init_this_cpu_offset() helper that's called before the usual
> primary/secondary start functions. For consistency, this is also used to
> re-initialize the per-cpu offset after the runtime per-cpu areas have
> been allocated (which can change CPU0's offset).
> 
> So that init_this_cpu_offset() isn't subject to any instrumentation that
> might consume the per-cpu offset, it is marked with noinstr, preventing
> instrumentation.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>

Reverting this commit on the top of today's linux-next fixed an issue that
Thunder X2 is unable to boot:

.config: https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config

EFI stub: Booting Linux Kernel...
EFI stub: EFI_RNG_PROTOCOL unavailable, KASLR will be disabled
EFI stub: Using DTB from configuration table
EFI stub: Exiting boot services and installing virtual address map...

It hangs here for more than 10 minutes even with "earlycon" before I gave up.
The reverting makes it boot again following by those lines almost immediately.

[    0.000000][    T0] Booting Linux on physical CPU 0x0000000000 [0x431f0af1]
[    0.000000][    T0] Linux version 5.9.0-rc8-next-20201008+ (gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), GNU ld version 2.30-79.el8) #6 SMP Thu Oct 8 20:57:40 EDT 2020
[    0.000000][    T0] efi: EFI v2.70 by American Megatrends
[    0.000000][    T0] efi: ESRT=0xf9224418 SMBIOS=0xfcca0000 SMBIOS 3.0=0xfcc90000 ACPI 2.0=0xf9720000 MEMRESERVE=0xfc965918 
[    0.000000][    T0] esrt: Reserving ESRT space from 0x00000000f9224418 to 0x00000000f9224450.
[    0.000000][    T0] ACPI: Early table checksum verification disabled
[    0.000000][    T0] ACPI: RSDP 0x00000000F9720000 000024 (v02 HPE   )
[    0.000000][    T0] ACPI: XSDT 0x00000000F9720028 0000DC (v01 HPE    ServerCL 01072009 AMI  00010013)
[    0.000000][    T0] ACPI: FACP 0x00000000F9720108 000114 (v06 HPE    ServerCL 01072009 AMI  00010013)
[    0.000000][    T0] ACPI: DSDT 0x00000000F9720220 000714 (v02 HPE    ServerCL 20150406 INTL 20170831)
[    0.000000][    T0] ACPI: FIDT 0x00000000F9720938 00009C (v01 HPE    ServerCL 01072009 AMI  00010013)
...

# lscpu
Architecture:        aarch64
Byte Order:          Little Endian
CPU(s):              224
On-line CPU(s) list: 0-223
Thread(s) per core:  4
Core(s) per socket:  28
Socket(s):           2
NUMA node(s):        2
Vendor ID:           Cavium
Model:               1
Model name:          ThunderX2 99xx
Stepping:            0x1
BogoMIPS:            400.00
L1d cache:           32K
L1i cache:           32K
L2 cache:            256K
L3 cache:            32768K
NUMA node0 CPU(s):   0-111
NUMA node1 CPU(s):   112-223
Flags:               fp asimd aes pmull sha1 sha2 crc32 atomics cpuid asimdrdm

> ---
>  arch/arm64/include/asm/cpu.h |  2 ++
>  arch/arm64/kernel/head.S     |  3 +++
>  arch/arm64/kernel/setup.c    | 12 ++++++------
>  arch/arm64/kernel/smp.c      | 13 ++++++++-----
>  4 files changed, 19 insertions(+), 11 deletions(-)
> 
> Since v1[1]:
> 
> * Fix typos
> * Rebase atop v5.9-rc4
> 
> Mark.
> 
> [1] https://lore.kernel.org/r/20200730163806.23053-1-mark.rutland@xxxxxxx
> 
> diff --git a/arch/arm64/include/asm/cpu.h b/arch/arm64/include/asm/cpu.h
> index 7faae6ff3ab4d..d9d60b18e8116 100644
> --- a/arch/arm64/include/asm/cpu.h
> +++ b/arch/arm64/include/asm/cpu.h
> @@ -68,4 +68,6 @@ void __init init_cpu_features(struct cpuinfo_arm64 *info);
>  void update_cpu_features(int cpu, struct cpuinfo_arm64 *info,
>  				 struct cpuinfo_arm64 *boot);
>  
> +void init_this_cpu_offset(void);
> +
>  #endif /* __ASM_CPU_H */
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 037421c66b147..2720e6ec68140 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -452,6 +452,8 @@ SYM_FUNC_START_LOCAL(__primary_switched)
>  	bl	__pi_memset
>  	dsb	ishst				// Make zero page visible to
> PTW
>  
> +	bl	init_this_cpu_offset
> +
>  #ifdef CONFIG_KASAN
>  	bl	kasan_early_init
>  #endif
> @@ -758,6 +760,7 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
>  	ptrauth_keys_init_cpu x2, x3, x4, x5
>  #endif
>  
> +	bl	init_this_cpu_offset
>  	b	secondary_start_kernel
>  SYM_FUNC_END(__secondary_switched)
>  
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 53acbeca4f574..fde4396418add 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -87,12 +87,6 @@ void __init smp_setup_processor_id(void)
>  	u64 mpidr = read_cpuid_mpidr() & MPIDR_HWID_BITMASK;
>  	set_cpu_logical_map(0, mpidr);
>  
> -	/*
> -	 * clear __my_cpu_offset on boot CPU to avoid hang caused by
> -	 * using percpu variable early, for example, lockdep will
> -	 * access percpu variable inside lock_release
> -	 */
> -	set_my_cpu_offset(0);
>  	pr_info("Booting Linux on physical CPU 0x%010lx [0x%08x]\n",
>  		(unsigned long)mpidr, read_cpuid_id());
>  }
> @@ -281,6 +275,12 @@ u64 cpu_logical_map(int cpu)
>  	return __cpu_logical_map[cpu];
>  }
>  
> +void noinstr init_this_cpu_offset(void)
> +{
> +	unsigned int cpu = task_cpu(current);
> +	set_my_cpu_offset(per_cpu_offset(cpu));
> +}
> +
>  void __init __no_sanitize_address setup_arch(char **cmdline_p)
>  {
>  	init_mm.start_code = (unsigned long) _text;
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 355ee9eed4dde..7714310fba226 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -192,10 +192,7 @@ asmlinkage notrace void secondary_start_kernel(void)
>  	u64 mpidr = read_cpuid_mpidr() & MPIDR_HWID_BITMASK;
>  	struct mm_struct *mm = &init_mm;
>  	const struct cpu_operations *ops;
> -	unsigned int cpu;
> -
> -	cpu = task_cpu(current);
> -	set_my_cpu_offset(per_cpu_offset(cpu));
> +	unsigned int cpu = smp_processor_id();
>  
>  	/*
>  	 * All kernel threads share the same mm context; grab a
> @@ -435,7 +432,13 @@ void __init smp_cpus_done(unsigned int max_cpus)
>  
>  void __init smp_prepare_boot_cpu(void)
>  {
> -	set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
> +	/*
> +	 * Now that setup_per_cpu_areas() has allocated the runtime per-cpu
> +	 * areas it is only safe to read the CPU0 boot-time area, and we must
> +	 * reinitialize the offset to point to the runtime area.
> +	 */
> +	init_this_cpu_offset();
> +
>  	cpuinfo_store_boot_cpu();
>  
>  	/*