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Date:   Tue, 23 Apr 2019 13:48:10 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Michael Chan <michael.chan@...adcom.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org,
        netdev@...r.kernel.org, linux-wireless@...r.kernel.org
Subject: Re: [PATCH v7 15/21] x86/split_lock: Add a sysfs interface to
 enable/disable split lock detection during run time

On Thu, Apr 18, 2019 at 08:41:30AM +0200, Thomas Gleixner wrote:
> On Wed, 17 Apr 2019, Fenghua Yu wrote:
> > On Thu, Apr 18, 2019 at 12:47:24AM +0200, Thomas Gleixner wrote:
> > > On Wed, 17 Apr 2019, Fenghua Yu wrote:
> > > 
> > > > The interface /sys/device/system/cpu/split_lock_detect is added
> > > > to allow user to control split lock detection and show current split
> > > > lock detection setting.
> > > > 
> > > > Writing [yY1] or [oO][nN] to the file enables split lock detection and
> > > > writing [nN0] or [oO][fF] disables split lock detection. Split lock
> > > > detection is enabled or disabled on all CPUs.
> > > > 
> > > > Reading the file returns current global split lock detection setting:
> > > > 0: disabled
> > > > 1: enabled
> > > 
> > > Again, You explain WHAT this patch does and still there is zero
> > > justification why this sysfs knob is needed at all. I still do not see any
> > > reason why this knob should exist.
> > 
> > An important application has split lock issues which are already discovered
> > and need to be fixed. But before the issues are fixed, sysadmin still wants to
> > run the application without rebooting the system, the sysfs knob can be useful
> > to turn off split lock detection. After the application is done, split lock
> > detection will be enabled again through the sysfs knob.
> 
> Are you sure that you are talking about the real world? I might buy the
> 'off' part somehow, but the 'on' part is beyond theoretical.
> 
> Even the 'off' part is dubious on a multi user machine. I personally would
> neither think about using the sysfs knob nor about rebooting the machine
> simply because I'd consider a lock operation accross a cacheline an malicious
> DoS attempt. Why would I allow that?
> 
> So in reality the sysadmin will either move the workload to a machine w/o
> the #AC magic or just tell the user to fix his crap.
> 
> > Without the sysfs knob, sysadmin has to reboot the system with kernel option
> > "no_split_lock_detect" to run the application before the split lock issues
> > are fixed.
> > 
> > Is this a valid justification why the sysfs knob is needed? If it is, I can
> > add the justification in the next version.
> 
> Why has this information not been in the changelog right away? I'm really
> tired of asking the same questions and pointing you to
> Documentation/process over and over.

So should I remove the sysfs knob patches in the next version?

Or add the following justification and still keep the sysfs knob patches?
"To workaround or debug a split lock issue, the administrator may need to
disable or enable split lock detection during run time without rebooting
the system."

Thanks.

-Fenghua

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