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

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Message-ID: <762682ba43a0468897ff5ddbf6633d58@AcuMS.aculab.com>
Date:   Thu, 25 Apr 2019 10:52:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Fenghua Yu' <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>
CC:     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

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.

AFAICT you've only fixed the ones that have actually faulted.
You've not even fixed the other ones in the same source files
as ones you've 'fixed'.

You need to do a code audit of all the in-tree kernel code
that uses locked operations - especially the 'bit' ones
since a lot of code casts the bitmap address - so it probably
isn't long aligned.

	David

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