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

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Message-ID: <7e3fd8ee-a17b-a469-22d9-4acf152e2527@intel.com>
Date:   Tue, 15 May 2018 09:30:51 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Alan Cox <alan@...ux.intel.com>, 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>
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 05/15/2018 09:26 AM, Alan Cox wrote:
>> Could you describe the performance degradation?  Why is this not a
>> "doctor it hurts when I do that" situation?
> They already know it hurts - the question is why and where.

I thought there were also cross-VM and cross-process concerns where
somebody unprivileged can make the whole system slow down.  They might
even be malicious, so this isn't something that *just* hurts one app or
one VM.

I think that's what I wanted to see hit the changelog.