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Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature
To: Dave Hansen <dave.hansen@intel.com>,
        Konrad Rzeszutek Wilk <konrad@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        linux-kernel@vger.kernel.org
References: <cover.1515204614.git.tim.c.chen@linux.intel.com>
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 <20180106144110.GA2592@localhost.localdomain>
 <742ed1d9-7210-8443-0373-5af74f193ab9@intel.com>
From: Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <7d2457b1-9cfb-f9b4-2d7c-69fd99492375@linux.intel.com>
Date: Sat, 6 Jan 2018 10:23:09 -0800
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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