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Subject: Re: [patch V4 part 1 14/36] x86/entry: Get rid of
 ist_begin/end_non_atomic()
To:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     x86@kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
References: <20200505131602.633487962@linutronix.de>
 <20200505134059.462640294@linutronix.de>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <e0037089-113c-d262-bae0-1beacf4198d8@oracle.com>
Date:   Wed, 6 May 2020 17:34:31 +0200
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On 5/5/20 3:16 PM, Thomas Gleixner wrote:
> This is completely overengineered and definitely not an interface which
> should be made available to anything else than this particular MCE case.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>   arch/x86/include/asm/traps.h   |    2 --
>   arch/x86/kernel/cpu/mce/core.c |    6 ++++--
>   arch/x86/kernel/traps.c        |   37 -------------------------------------
>   3 files changed, 4 insertions(+), 41 deletions(-)

Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>

alex.

> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -120,8 +120,6 @@ asmlinkage void smp_irq_move_cleanup_int
>   
>   extern void ist_enter(struct pt_regs *regs);
>   extern void ist_exit(struct pt_regs *regs);
> -extern void ist_begin_non_atomic(struct pt_regs *regs);
> -extern void ist_end_non_atomic(void);
>   
>   #ifdef CONFIG_VMAP_STACK
>   void __noreturn handle_stack_overflow(const char *message,
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -1352,13 +1352,15 @@ void notrace do_machine_check(struct pt_
>   
>   	/* Fault was in user mode and we need to take some action */
>   	if ((m.cs & 3) == 3) {
> -		ist_begin_non_atomic(regs);
> +		/* If this triggers there is no way to recover. Die hard. */
> +		BUG_ON(!on_thread_stack() || !user_mode(regs));
>   		local_irq_enable();
> +		preempt_enable();
>   
>   		if (kill_it || do_memory_failure(&m))
>   			force_sig(SIGBUS);
> +		preempt_disable();
>   		local_irq_disable();
> -		ist_end_non_atomic();
>   	} else {
>   		if (!fixup_exception(regs, X86_TRAP_MC, error_code, 0))
>   			mce_panic("Failed kernel mode recovery", &m, msg);
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -117,43 +117,6 @@ void ist_exit(struct pt_regs *regs)
>   		rcu_nmi_exit();
>   }
>   
> -/**
> - * ist_begin_non_atomic() - begin a non-atomic section in an IST exception
> - * @regs:	regs passed to the IST exception handler
> - *
> - * IST exception handlers normally cannot schedule.  As a special
> - * exception, if the exception interrupted userspace code (i.e.
> - * user_mode(regs) would return true) and the exception was not
> - * a double fault, it can be safe to schedule.  ist_begin_non_atomic()
> - * begins a non-atomic section within an ist_enter()/ist_exit() region.
> - * Callers are responsible for enabling interrupts themselves inside
> - * the non-atomic section, and callers must call ist_end_non_atomic()
> - * before ist_exit().
> - */
> -void ist_begin_non_atomic(struct pt_regs *regs)
> -{
> -	BUG_ON(!user_mode(regs));
> -
> -	/*
> -	 * Sanity check: we need to be on the normal thread stack.  This
> -	 * will catch asm bugs and any attempt to use ist_preempt_enable
> -	 * from double_fault.
> -	 */
> -	BUG_ON(!on_thread_stack());
> -
> -	preempt_enable_no_resched();
> -}
> -
> -/**
> - * ist_end_non_atomic() - begin a non-atomic section in an IST exception
> - *
> - * Ends a non-atomic section started with ist_begin_non_atomic().
> - */
> -void ist_end_non_atomic(void)
> -{
> -	preempt_disable();
> -}
> -
>   int is_valid_bugaddr(unsigned long addr)
>   {
>   	unsigned short ud;
>