Re: [RESEND PATCH v2 1/3] x86/fred: Allow variable-sized event frame

[Xin Li <xin@zytor.com>]

On 3/18/2025 6:53 AM, Brian Gerst wrote:
>> -#define INIT_THREAD_INFO(tsk)                  \
>> -{                                              \
>> -       .flags          = 0,                    \
>> +#define INIT_THREAD_INFO(tsk)                                          \
>> +{                                                                      \
>> +       .flags          = 0,                                            \
>> +       .user_pt_regs   = (struct pt_regs *)TOP_OF_INIT_STACK - 1,      \
> 
> Use __top_init_kernel_stack here.

Will do.

> 
>>   }
>>
>>   #else /* !__ASSEMBLER__ */
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index 91f6ff618852..58c1cd4ca60a 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -108,6 +108,28 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>>          return 0;
>>   }
>>
>> +/*
>> + * Initialize thread_info.user_pt_regs for IDT event delivery.
>> + *
>> + * For IDT user level event delivery, a pt_regs structure is pushed by both
>> + * hardware and software and always resides at a fixed offset from top of
>> + * current task kernel stack, thus thread_info.user_pt_regs is a per-task
>> + * constant and NEVER changes after initialization.
>> + *
>> + * While for FRED user level event delivery, user_pt_regs is updated in
>> + * fred_entry_from_user() immediately after user level event delivery.
>> + *
>> + * Note: thread_info.user_pt_regs of the init task is initialized at build
>> + * time.
>> + */
>> +void arch_init_user_pt_regs(struct task_struct *tsk)
>> +{
>> +       unsigned long top_of_stack = (unsigned long)task_stack_page(tsk) + THREAD_SIZE;
>> +
>> +       top_of_stack -= TOP_OF_KERNEL_STACK_PADDING;
>> +       tsk->thread_info.user_pt_regs = (struct pt_regs *)top_of_stack - 1;
>> +}
> 
> Can this be put into arch_dup_task_struct() instead of creating another hook?

I wanted to do it at the beginning but task stack is no longer part of
the task_struct on x86.  Make sense?

Thanks!
     Xin