Re: [PATCH v2] split up lockdep and syscall related functionality in generic entry code

[Thomas Gleixner]

On Tue, Dec 01 2020 at 15:27, Sven Schnelle wrote:
> __do_syscall is the function which gets called by low level entry.S code:
>
> void noinstr __do_syscall(struct pt_regs *regs)
> {
>       enter_from_user_mode(regs);     /* sets lockdep state, and other 
> initial stuff */
>
>       /*
>        * functions that need to run with irqs disabled,
>        * but lockdep state and other stuff set up
>        */
>       memcpy(&regs->gprs[8], S390_lowcore.save_area_sync, 8 * sizeof(unsigned 
> long));
>       memcpy(&regs->int_code, &S390_lowcore.svc_ilc, sizeof(regs->int_code));
>       regs->psw = S390_lowcore.svc_old_psw;

As __do_syscall() is marked noinstr you want to add

        instrumentation_begin();
>
>       update_timer_sys();
>
>       local_irq_enable();
>
>       regs->orig_gpr2 = regs->gprs[2];
>
>       do {
>               regs->flags = _PIF_SYSCALL;
>               do_syscall(regs);
>       } while (test_pt_regs_flag(regs, PIF_SYSCALL_RESTART));

        instrumentation_end();

for two reasons:

    - it clearly documents the boundaries in the code

    - it will make objtool happy.

      I know it does not have s390 support yet, but I only can recommend
      to add that. It's annoying to analyze all the spots it complains
      about violating the noinstr rules, but it's way more reliable than
      human inspection.
      

>       exit_to_user_mode();
> }

Thanks,

        tglx