linux-kernel - Re: [RFC PATCH v3 8/8] arm64: Ensure the secondary CPUs have safe ASIDBits size

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Date:	Tue, 15 Dec 2015 12:02:30 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Suzuki K. Poulose" <suzuki.poulose@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	mark.rutland@....com, catalin.marinas@....com, marc.zyngier@....com
Subject: Re: [RFC PATCH v3 8/8] arm64: Ensure the secondary CPUs have safe
 ASIDBits size

On Wed, Dec 09, 2015 at 09:57:19AM +0000, Suzuki K. Poulose wrote:
> The ID_AA64MMFR0_EL1:ASIDBits determines the size of the mm context
> id and is used in the early boot to make decisions. The value is
> picked up from the Boot CPU and cannot be delayed until other CPUs
> are up. If a secondary CPU has a smaller size than that of the Boot
> CPU, things will break horribly and the usual SANITY check is not good
> enough to prevent the system from crashing. So, crash the system with
> enough information.
> 
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com>
> ---
>  arch/arm64/include/asm/mmu_context.h |    2 ++
>  arch/arm64/kernel/cpufeature.c       |    2 ++
>  arch/arm64/mm/context.c              |   16 ++++++++++++++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index 2416578..bd8a0b9 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -147,4 +147,6 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  #define deactivate_mm(tsk,mm)	do { } while (0)
>  #define activate_mm(prev,next)	switch_mm(prev, next, NULL)
>  
> +void verify_cpu_asid_bits(void);
> +
>  #endif
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index e63da0f..f7bcd30 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -24,6 +24,7 @@
>  #include <asm/cpu.h>
>  #include <asm/cpufeature.h>
>  #include <asm/cpu_ops.h>
> +#include <asm/mmu_context.h>
>  #include <asm/processor.h>
>  #include <asm/sysreg.h>
>  
> @@ -829,6 +830,7 @@ static u64 __raw_read_system_reg(u32 sys_id)
>   */
>  static void check_early_cpu_features(void)
>  {
> +	verify_cpu_asid_bits();
>  }
>  
>  /*
> diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> index 643bf4b..b945bb4 100644
> --- a/arch/arm64/mm/context.c
> +++ b/arch/arm64/mm/context.c
> @@ -24,6 +24,7 @@
>  
>  #include <asm/cpufeature.h>
>  #include <asm/mmu_context.h>
> +#include <asm/smp.h>
>  #include <asm/tlbflush.h>
>  
>  static u32 asid_bits;
> @@ -62,6 +63,21 @@ static u32 get_cpu_asid_bits(void)
>  	return asid;
>  }
>  
> +/* Check if the current cpu's ASIDBits is compatible with asid_bits */
> +void verify_cpu_asid_bits(void)
> +{
> +	u32 asid = get_cpu_asid_bits();
> +
> +	if (asid < asid_bits) {
> +		/* Differing ASIDBits is dangerous, crash the system */

This comment isn't much use. How about:

  /*
   * We cannot decrease the ASID size at runtime, so panic if we support
   * fewer ASID bits than the boot CPU.
   */

> +		pr_crit("CPU%d: has smaller ASIDBits(%u) than the one in use (%u)\n",
> +				smp_processor_id(), asid, asid_bits);

"CPU%d: smaller ASID size (%d) than boot CPU (%u)\n"

> +		update_cpu_boot_status(CPU_PANIC_KERNEL);
> +		isb();

Why?

> +		while(1);

We probably want the usuale wfe/wfi trick here. Perhaps we should have
an inline function for that.

Will
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