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Date:   Thu, 21 Nov 2019 15:14:11 -0800
From:   Fenghua Yu <fenghua.yu@intel.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tony Luck <tony.luck@intel.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 5/6] x86/split_lock: Handle #AC exception for split
 lock
Message-ID: <20191121231410.GD199273@romley-ivt3.sc.intel.com>
References: <1574297603-198156-6-git-send-email-fenghua.yu@intel.com>
 <5BDDAE0C-2D31-4779-B3A0-5BF206FF3E50@amacapital.net>
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On Thu, Nov 21, 2019 at 02:10:38PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@intel.com> wrote:
> > 
> > +    if (!user_mode(regs) && split_lock_detect_enabled)
> > +        panic("Split lock detected\n");
> 
> NAK.
> 
> 1. Don’t say “split lock detected” if you don’t know that you detected a split lock.  Or is this genuinely the only way to get #AC from kernel mode?

Intel hardware design team confirmed that the only reason for #AC in ring 0 is
split lock.

> 
> 2. Don’t panic. Use die() just like every other error where nothing is corrupted.

Ok. Will change to die() which provides all the trace information and
allow multiple split lock in one boot.

> 
> And maybe instead turn off split lock detection and print a stack trace instead.  Then the kernel is even more likely to survive to log something useful.

How about we just use simple policy die() in this patch set to allow
detect and debug split lock issues and extend the code base to handle
split lock with different policies (panic, disable split lock, maybe other
options) in the future?

Thanks.

-Fenghua