linux-kernel - Re: [PATCH 0/15] x86/split_lock: Enable #AC exception for split locked accesses

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Date:   Tue, 15 May 2018 08:10:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        Alan Cox <alan@...ux.intel.com>
Cc:     x86 <x86@...nel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/15] x86/split_lock: Enable #AC exception for split
 locked accesses

On 05/14/2018 11:52 AM, Fenghua Yu wrote:
> When executing an atomic instruction, sometimes the data access crosses
> the cache line boundary. This causes performance degradation. Intel
> introduces mechanism to detect such split lock issue via alignment check
> fault.

This is still a pretty sparse description.

Could you describe the performance degradation?  Why is this not a
"doctor it hurts when I do that" situation?

Could you also give us *some* idea which CPUs this will show up on?  Is
it a one-off feature, or will it be available widely?

> Since kernel doesn't know when SMI comes, it's impossible for kernel
> to disable split lock #AC before entering SMI. So SMI handler may
> inherit kernel's split lock setting and kernel tester may end up
> debug split lock issues in SMI.

What's a "kernel tester"?