linux-kernel - Re: [PATCH v17] x86/split_lock: Enable split lock detection by kernel

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Date:   Mon, 3 Feb 2020 16:04:49 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Mark D Rustad <mrustad@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v17] x86/split_lock: Enable split lock detection by kernel

On Sun, Jan 26, 2020 at 12:05:35PM -0800, Luck, Tony wrote:

...

> +bool handle_user_split_lock(struct pt_regs *regs, long error_code)

No reason to take the error code unless there's a plan to use it.

> +{
> +	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> +		return false;

Any objection to moving the EFLAGS.AC up to do_alignment_check()?  And
take "unsigned long rip" instead of @regs?

That would allow KVM to reuse handle_user_split_lock() for guest faults
without any changes (other than exporting).

E.g. do_alignment_check() becomes:

	if (!(regs->flags & X86_EFLAGS_AC) && handle_user_split_lock(regs->ip))
		return;

> +
> +	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> +			    current->comm, current->pid, regs->ip);
> +
> +	/*
> +	 * Disable the split lock detection for this task so it can make
> +	 * progress and set TIF_SLD so the detection is re-enabled via
> +	 * switch_to_sld() when the task is scheduled out.
> +	 */
> +	__sld_msr_set(false);
> +	set_tsk_thread_flag(current, TIF_SLD);
> +	return true;
> +}

...

> +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
> +{
> +	char *str = "alignment check";
> +
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +
> +	if (notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_AC, SIGBUS) == NOTIFY_STOP)
> +		return;
> +
> +	if (!user_mode(regs))
> +		die("Split lock detected\n", regs, error_code);
> +
> +	local_irq_enable();
> +
> +	if (handle_user_split_lock(regs, error_code))
> +		return;
> +
> +	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
> +		error_code, BUS_ADRALN, NULL);
> +}
> +
>  #ifdef CONFIG_VMAP_STACK
>  __visible void __noreturn handle_stack_overflow(const char *message,
>  						struct pt_regs *regs,
> -- 
> 2.21.1
>