linux-kernel - Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Sat, 6 Jan 2018 11:22:52 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On 01/06/2018 09:41 AM, Van De Ven, Arjan wrote:
>>>>  .macro DISABLE_IBRS
>>>> -	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
>>>> +	testl	$1, dynamic_ibrs
>>> On every system call we end up hammering on this 'dynamic_ibrs'
>>> variable. And it looks like it can be flipped via the IPI mechanism.
>>>
>>> Would it make sense for this to be per-cpu?
>>
>> It's probably better to either just make it __read_mostly or get the
>> static branches that folks were suggesting actually working.
> 
> I still wonder if this isn't just better as a boot command line

It's simpler that way.  But, ideally, we want to make it runtime
switchable to match the implementation in the distros.