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

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Message-ID: <0575AF4FD06DD142AD198903C74E1CC87A56C6FE@ORSMSX103.amr.corp.intel.com>
Date:   Sat, 6 Jan 2018 17:41:44 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     "Hansen, Dave" <dave.hansen@...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

> >>  .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