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

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Message-ID: <877dq1f75q.fsf@nanos.tec.linutronix.de>
Date:   Wed, 02 Dec 2020 00:17:21 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] split up lockdep and syscall related functionality in generic entry code

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