Re: [PATCH -next v4 02/19] arm64: entry: Refactor the entry and exit for exceptions from EL1

[Jinjie Ruan <ruanjinjie@xxxxxxxxxx>]

On 2024/10/29 22:33, Mark Rutland wrote:
> On Fri, Oct 25, 2024 at 06:06:43PM +0800, Jinjie Ruan wrote:
>> These changes refactor the entry and exit routines for the exceptions
>> from EL1. They store the RCU and lockdep state in a struct
>> irqentry_state variable on the stack, rather than recording them
>> in the fields of pt_regs, since it is safe enough for these context.
> 
> In general, please descirbe *why* we want to make the change first, e.g.
> 
> | The generic entry code uses irqentry_state_t to track lockdep and RCU
> | state across exception entry and return. For historical reasons, arm64
> | embeds similar fields within its pt_regs structure.
> |
> | In preparation for moving arm64 over to the generic entry code, pull
> | these fields out of arm64's pt_regs, and use a seperate structure,
> | matching the style of the generic entry code.
> 
>> Before:
>>      struct pt_regs {
>>              ...
>>              u64 lockdep_hardirqs;
>>              u64 exit_rcu;
>>      }
>>
>>      enter_from_kernel_mode(regs);
>>      ...
>>      exit_to_kernel_mode(regs);
>>
>> After:
>>      typedef struct irqentry_state {
>>              union {
>>                      bool    exit_rcu;
>>                      bool    lockdep;
>>              };
>>      } irqentry_state_t;
>>
>>      irqentry_state_t state = enter_from_kernel_mode(regs);
>>      ...
>>      exit_to_kernel_mode(regs, state);
> 
> I don't think this part is necessary.

Thank you, will remove it and explain why.

> 
>>
>> No functional changes.
>>
>> Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/ptrace.h  |  11 ++-
>>  arch/arm64/kernel/entry-common.c | 129 +++++++++++++++++++------------
>>  2 files changed, 85 insertions(+), 55 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/ptrace.h 
>> b/arch/arm64/include/asm/ptrace.h
>> index 3e5372a98da4..5156c0d5fa20 100644
>> --- a/arch/arm64/include/asm/ptrace.h
>> +++ b/arch/arm64/include/asm/ptrace.h
>> @@ -149,6 +149,13 @@ static inline unsigned long pstate_to_compat_psr(const 
>> unsigned long pstate)
>>      return psr;
>>  }
>>  
>> +typedef struct irqentry_state {
>> +    union {
>> +            bool    exit_rcu;
>> +            bool    lockdep;
>> +    };
>> +} irqentry_state_t;
> 
> AFAICT this can be moved directly into arch/arm64/kernel/entry-common.c.
> 
>> +
>>  /*
>>   * This struct defines the way the registers are stored on the stack during 
>> an
>>   * exception. struct user_pt_regs must form a prefix of struct pt_regs.
>> @@ -169,10 +176,6 @@ struct pt_regs {
>>  
>>      u64 sdei_ttbr1;
>>      struct frame_record_meta stackframe;
>> -
>> -    /* Only valid for some EL1 exceptions. */
>> -    u64 lockdep_hardirqs;
>> -    u64 exit_rcu;
>>  };
>>  
>>  /* For correct stack alignment, pt_regs has to be a multiple of 16 bytes. */
>> diff --git a/arch/arm64/kernel/entry-common.c 
>> b/arch/arm64/kernel/entry-common.c
>> index c547e70428d3..68a9aecacdb9 100644
>> --- a/arch/arm64/kernel/entry-common.c
>> +++ b/arch/arm64/kernel/entry-common.c
>> @@ -36,29 +36,36 @@
>>   * This is intended to match the logic in irqentry_enter(), handling the 
>> kernel
>>   * mode transitions only.
>>   */
>> -static __always_inline void __enter_from_kernel_mode(struct pt_regs *regs)
>> +static __always_inline irqentry_state_t __enter_from_kernel_mode(struct 
>> pt_regs *regs)
>>  {
>> -    regs->exit_rcu = false;
>> +    irqentry_state_t ret = {
>> +            .exit_rcu = false,
>> +    };
> 
> I realise that the generic entry code calls this 'ret' in
> irqentry_enter() and similar, but could we please use 'state'
> consistently in the arm64 code?
> 
> [...]
> 
>>  /*
>> @@ -190,9 +199,11 @@ asmlinkage void noinstr asm_exit_to_user_mode(struct 
>> pt_regs *regs)
>>   * mode. Before this function is called it is not safe to call regular 
>> kernel
>>   * code, instrumentable code, or any code which may trigger an exception.
>>   */
>> -static void noinstr arm64_enter_nmi(struct pt_regs *regs)
>> +static noinstr irqentry_state_t arm64_enter_nmi(struct pt_regs *regs)
>>  {
>> -    regs->lockdep_hardirqs = lockdep_hardirqs_enabled();
>> +    irqentry_state_t irq_state;
> 
> Likewise, please use 'state' rather than 'irq_state'.
> 
> In future we should probably have a separate structure for the NMI
> paths, and get rid of the union, which would avoid the possiblity of
> using mismatched helpers.
> 
> Mark.
> 
>