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

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Date:   Wed, 22 Jan 2020 23:24:34 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
CC:     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

>> +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

>> +	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