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 10:23:09 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>
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>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        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 09:33 AM, Dave Hansen wrote:
> On 01/06/2018 06:41 AM, Konrad Rzeszutek Wilk 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.
> 

dynamic_ibrs is indeed declared __read_mostly

+unsigned int dynamic_ibrs __read_mostly;
+EXPORT_SYMBOL_GPL(dynamic_ibrs);

Tim