linux-kernel - RE: [PATCH v8 13/15] x86/split_lock: Enable split lock detection by default

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Date:   Thu, 25 Apr 2019 12:58:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>
cc:     'Fenghua Yu' <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Michael Chan <michael.chan@...adcom.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: RE: [PATCH v8 13/15] x86/split_lock: Enable split lock detection by
 default

On Thu, 25 Apr 2019, David Laight wrote:

> From:  Fenghua Yu
> > Sent: 24 April 2019 20:33
> > A split locked access locks bus and degrades overall memory access
> > performance. When split lock detection feature is enumerated, enable
> > the feature by default by writing 1 to bit 29 in MSR TEST_CTL to find
> > any split lock issue.
> 
> You can't enable this by default until ALL the known potentially
> misaligned locked memory operations have been fixed.

Errm? The result will be a WARN_ON() printed and no further damage. It's
not making anything worse than it is now. In fact we just should add a

    WARN_ON_ONCE(!aligned_to_long(p)) to all the xxx_bit() operations.

so we catch them even when they do not trigger that #AC thingy.

Thanks,

	tglx