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

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Date:   Wed, 22 Jan 2020 19:45:08 -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 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.

> 
> >> +	if (!split_lock_detect_enabled())
> >> +		return;
> >
> > This misses one comment from Sean [1] that this check should be dropped,
> > otherwise user-space alignment check via EFLAGS.AC will get ignored when
> > split lock detection is disabled.
> 
> Ah yes. Good catch.  Will fix.
> 
> Thanks for the review.
> 
> -Tony