Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Wed, 22 Jan 2020 19:45:08 -0500
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v12] x86/split_lock: Enable split lock detection by kernel
Message-ID: <20200123004507.GA2403906@rani.riverdale.lan>
References: <20191123003056.GA28761@agluck-desk2.amr.corp.intel.com>
 <20191125161348.GA12178@linux.intel.com>
 <20191212085948.GS2827@hirez.programming.kicks-ass.net>
 <20200110192409.GA23315@agluck-desk2.amr.corp.intel.com>
 <20200114055521.GI14928@linux.intel.com>
 <20200115222754.GA13804@agluck-desk2.amr.corp.intel.com>
 <20200115225724.GA18268@linux.intel.com>
 <20200122185514.GA16010@agluck-desk2.amr.corp.intel.com>
 <20200122224245.GA2331824@rani.riverdale.lan>
 <3908561D78D1C84285E8C5FCA982C28F7F54887A@ORSMSX114.amr.corp.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <3908561D78D1C84285E8C5FCA982C28F7F54887A@ORSMSX114.amr.corp.intel.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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