linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Date:   Wed, 10 Jan 2018 13:27:55 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, Jan 10, 2018 at 01:20:45PM +0100, Andrea Arcangeli wrote:
> On Wed, Jan 10, 2018 at 12:12:53PM +0000, David Woodhouse wrote:
> > IBRS is like a barrier. You must write it between the 'problematic'
> > loading of the branch targets, and the kernel code which might be
> > affected.
> > 
> > You cannot, on current hardware, merely set it once and forget about
> > it. That is not sufficient.
> 
> I think you've got it all wrong...

On a side note, I hope once you get it right, the debugfs tunable will
suddenly look less horrid.