linux-kernel - Re: [PATCH v12] x86/split_lock: Enable split lock detection by kernel

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Message-ID: <20200123042132.GA2448175@rani.riverdale.lan>
Date:   Wed, 22 Jan 2020 23:21:33 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v12] x86/split_lock: Enable split lock detection by kernel

On Wed, Jan 22, 2020 at 05:23:17PM -0800, Luck, Tony wrote:
> On Wed, Jan 22, 2020 at 07:45:08PM -0500, Arvind Sankar wrote:
> > On Wed, Jan 22, 2020 at 11:24:34PM +0000, Luck, Tony wrote:
> > > >> +static enum split_lock_detect_state sld_state = sld_warn;
> > > >> +
> > > >
> > > > This sets sld_state to sld_warn even on CPUs that don't support
> > > > split-lock detection. split_lock_init will then try to read/write the
> > > > MSR to turn it on. Would it be better to initialize it to sld_off and
> > > > set it to sld_warn in split_lock_setup instead, which is only called if
> > > > the CPU supports the feature?
> > > 
> > > I've lost some bits of this patch series somewhere along the way :-(  There
> > > was once code to decide whether the feature was supported (either with
> > > x86_match_cpu() for a couple of models, or using the architectural test
> > > based on some MSR bits.  I need to dig that out and put it back in. Then
> > > stuff can check X86_FEATURE_SPLIT_LOCK before wandering into code
> > > that messes with MSRs
> > 
> > That code is still there (cpu_set_core_cap_bits). The issue is that with
> > the initialization here, nothing ever sets sld_state to sld_off if the
> > feature isn't supported.
> > 
> > v10 had a corresponding split_lock_detect_enabled that was
> > 0-initialized, but Peter's patch as he sent out had the flag initialized
> > to sld_warn.
> 
> Ah yes. Maybe the problem is that split_lock_init() is only
> called on systems that support split loc detect, while we call
> split_lock_init() unconditionally.

It was unconditional in v10 too?

> 
> What if we start with sld_state = sld_off, and then have split_lock_setup
> set it to either sld_warn, or whatever the user chose on the command
> line.  Patch below (on top of patch so you can see what I'm saying,
> but will just merge it in for next version.

Yep, that's what I suggested.

> 
> -Tony
> 
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 7478bebcd735..b6046ccfa372 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -39,7 +39,13 @@ enum split_lock_detect_state {
>  	sld_fatal,
>  };
>  
> -static enum split_lock_detect_state sld_state = sld_warn;
> +/*
> + * Default to sld_off because most systems do not support
> + * split lock detection. split_lock_setup() will switch this
> + * to sld_warn, and then check to see if there is a command
> + * line override.
> + */
> +static enum split_lock_detect_state sld_state = sld_off;
>  
>  /*
>   * Just in case our CPU detection goes bad, or you have a weird system,
> @@ -1017,10 +1023,11 @@ static inline bool match_option(const char *arg, int arglen, const char *opt)
>  
>  static void __init split_lock_setup(void)
>  {
> -	enum split_lock_detect_state sld = sld_state;
> +	enum split_lock_detect_state sld;

This is bike-shedding, but initializing sld = sld_warn here would have
been enough with no other changes to the patch I think?