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Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature
To: Konrad Rzeszutek Wilk <konrad@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
References: <cover.1515204614.git.tim.c.chen@linux.intel.com>
 <b77c0cfc2d50ebdaa721f95012fd735cc374f04c.1515204614.git.tim.c.chen@linux.intel.com>
 <20180106144110.GA2592@localhost.localdomain>
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
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <742ed1d9-7210-8443-0373-5af74f193ab9@intel.com>
Date: Sat, 6 Jan 2018 09:33:17 -0800
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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.