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Date:   Fri, 22 Nov 2019 21:23:12 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Ingo Molnar <mingo@kernel.org>, Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
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On Fri, Nov 22, 2019 at 09:22:46AM -0800, Luck, Tony wrote:
> On Fri, Nov 22, 2019 at 04:27:15PM +0100, Peter Zijlstra wrote:
> > +void handle_user_split_lock(struct pt_regs *regs, long error_code)
> > +{
> > +	if (sld_state == sld_fatal)
> > +		return false;
> > +
> > +	pr_alert("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> > +		 current->comm, current->pid, regs->ip);
> > +
> > +	__sld_set_msr(false);
> > +	set_tsk_thread_flag(current, TIF_CLD);
> > +	return true;
> > +}
> 
> I think you need an extra check in here. While a #AC in the kernel
> is an indication of a split lock. A user might have enabled alignment
> checking and so this #AC might not be from a split lock.
> 
> I think the extra code if just to change that first test to:
> 
> 	if ((regs->eflags & X86_EFLAGS_AC) || sld_fatal)

Indeed.