linux-kernel - Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

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Date:   Sat, 25 Jan 2020 14:41:33 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Luck\, Tony" <tony.luck@...el.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        "Christopherson\, Sean J" <sean.j.christopherson@...el.com>,
        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 v15] x86/split_lock: Enable split lock detection by kernel

Tony,

"Luck, Tony" <tony.luck@...el.com> writes:
> +
> +void switch_to_sld(struct task_struct *prev, struct task_struct *next)
> +{
> +	bool prevflag = test_tsk_thread_flag(prev, TIF_SLD);
> +	bool nextflag = test_tsk_thread_flag(next, TIF_SLD);
> +
> +	/*
> +	 * If we are switching between tasks that have the same
> +	 * need for split lock checking, then the MSR is (probably)
> +	 * right (modulo the other thread messing with it.
> +	 * Otherwise look at whether the new task needs split
> +	 * lock enabled.
> +	 */
> +	if (prevflag != nextflag)
> +		__sld_msr_set(nextflag);
> +}
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 839b5244e3b7..b34d359c4e39 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -650,6 +650,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
>  		/* Enforce MSR update to ensure consistent state */
>  		__speculation_ctrl_update(~tifn, tifn);
>  	}
> +
> +	switch_to_sld(prev_p, next_p);

This really wants to follow the logic of the other TIF checks.

        if ((tifp ^ tifn) & _TIF_SLD)
        	switch_to_sld(tifn);

and

void switch_to_sld(tifn)
{
        __sld_msr_set(tif & _TIF_SLD);
}

That reuses tifp, tifn which are ready to consume there and calls only
out of line when the bits differ. The xor/and combo turned out to result
in the most efficient code.

Thanks,

        tglx