linux-kernel - Re: [PATCH v6 13/20] x86/split_lock: Enable split lock detection by default

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Date:   Wed, 10 Apr 2019 05:35:46 -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>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Michael Chan <michael.chan@...adcom.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v6 13/20] x86/split_lock: Enable split lock detection by
 default

On Wed, Apr 10, 2019 at 08:31:31AM +0200, Thomas Gleixner wrote:
> On Tue, 9 Apr 2019, Fenghua Yu wrote:
> > On Thu, Apr 04, 2019 at 08:07:57PM +0200, Thomas Gleixner wrote:
> > > On Wed, 3 Apr 2019, Fenghua Yu wrote:
> > > >  static void early_init_intel(struct cpuinfo_x86 *c)
> > > >  {
> > > >  	u64 misc_enable;
> > > >  
> > > > +	init_split_lock_detect(c);
> > > 
> > > so we have in early boot:
> > > 
> > > 	early_cpu_init()
> > > 	  early_identify_cpu()
> > > 	    this_cpu->c_early_init(c)
> > > 	      early_init_intel() {
> > > 	        init_split_lock_detect();
> > > 	      }	
> > >             ....
> > >             cpu_set_core_cap_bits(c)
> > > 	       set(FEATURE_SPLIT_LOCK)
> > > 
> > > I don't have to understand how init_split_lock_detect() will magically see
> > > the feature bit which gets set afterwards, right? 
> > 
> > early_init_intel() is called twice on the boot CPU. Besides it's called
> > in earl_cpu_init(), it's also called in:
> > 	identify_boot_cpu()
> > 		identify_cpu()
> > 			init_intel()
> > 				early_init_intel()
> > 					init_split_lock_detect();
> > 
> > It's true that init_split_lock_detect() doesn't see the feature bit when
> > it's called for the first time in early_cpu_init(). But it sees the feature
> > bit when it's called for the second time in identify_boot_cpu().
> 
> That's hideous, really. 
> 
> > So is init_split_lock_detect() in the right place?
> 
> You're not seriously asking that?
> 
> It's obviously not the right place. We are not placing calls at random
> points just because they happen to work by chance.

Is it OK to put init_split_lock_detect(c) after early_init_intel() in
init_intel()? X86_FEATURE_SPLIT_LOCK_DETECT is available now and
init_split_lock_detec() is called only once on each CPU.

@@ -746,6 +749,8 @@ static void init_intel(struct cpuinfo_x86 *c)
 {
        early_init_intel(c);
 
+       init_split_lock_detect(c);
+
        intel_workarounds(c);

Thanks.

-Fenghua