linux-kernel - Re: [PATCH v8 13/15] x86/split_lock: Enable split lock detection by default

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Date:   Mon, 6 May 2019 14:39:49 -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 v8 13/15] x86/split_lock: Enable split lock detection by
 default

On Thu, Apr 25, 2019 at 09:50:20AM +0200, Thomas Gleixner wrote:
> On Wed, 24 Apr 2019, Fenghua Yu wrote:
> >  
> > +static void split_lock_update_msr(void)
> > +{
> > +	/* Enable split lock detection */
> > +	msr_set_bit(MSR_TEST_CTL, TEST_CTL_SPLIT_LOCK_DETECT_SHIFT);
> > +	this_cpu_or(msr_test_ctl_cache, TEST_CTL_SPLIT_LOCK_DETECT);
> 
> I'm pretty sure, that I told you to utilize the cache proper. Again:
> 
> > > Nothing in this file initializes msr_test_ctl_cache explicitely. Register
> > > caching always requires to read the register and store it in the cache
> > > before doing anything with it. Nothing guarantees that all bits in that MSR
> > > are 0 by default forever.
> > >
> > > And once you do that _before_ calling split_lock_update_msr() then you can
> > > spare the RMW in that function.
> 
> So you managed to fix the initializaiton part, but then you still do a
> pointless RMW.

Ok. I see. msr_set_bit() is a RMW operation.

So is the following the right code to update msr and cache variable?

+static void split_lock_update_msr(void)
+{
+   /* Enable split lock detection */
+   this_cpu_or(msr_test_ctl_cache, TEST_CTL_SPLIT_LOCK_DETECT);
+   wrmsrl(MSR_TEST_CTL, msr_test_ctl_cache);

Thanks.

-Fenghua