linux-kernel - Re: [PATCH 09/15] x86/split_lock: Explicitly enable or disable #AC for split locked accesses

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Date:   Tue, 15 May 2018 09:15:16 -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 09/15] x86/split_lock: Explicitly enable or disable #AC
 for split locked accesses

On 05/14/2018 11:52 AM, Fenghua Yu wrote:
> By default, we don't set or clear the bit 29 in TEST_CTL MSR 0x33 and
> the bit is inherited from BIOS/hardware setting.
> 
> The kernel parameter "split_lock_ac=on/off" explicitly sets or clears
> the bit during boot time.

The more I think about this...  Why do we need this at boot anyway?
Surely boot-time kernel code can't cause performance issues in the same
way that untrusted repeated userspace can.  Why don't we just let
userspace turn this on?