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

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Message-ID: <20180110151317.GI9417@redhat.com>
Date:   Wed, 10 Jan 2018 16:13:17 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Jiri Kosina <jikos@...nel.org>,
        "asit.k.mallick" <asit.k.mallick@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, Jan 10, 2018 at 06:59:54AM -0800, Dave Hansen wrote:
> On 01/10/2018 06:10 AM, Andrea Arcangeli wrote:
> > Tim and Dave please comment too, Tim you originally wrote that code
> > that leaves IBRS always on and never toggles it in the kernel entry
> > point so you must know full well if Arjan is correct that you must
> > toggle IBRS every time you enter kernel and in turn ibrs_enabled 2
> > isn't valid mode.
> 
> Hi Andrea,
> 
> The "writing IBRS=1 acts as a barrier when it is already IBRS=1"
> behavior is something which I misunderstood in the past.  Thanks, Arjan,
> for clearing it up.

"writing IBRS=1 acts as a barrier when it is already IBRS=1" would
have been much clearer wording frankly. IBPB is IBP "Barrier", but
also IBRS is a barrier, no problem :).

So we'll add a dummy IBRS write to SPEC_CTRL in kernel entry and
vmexit so that it is compliant with all released microcodes that may
require it, also when ibrs_enabled is 2. Can you confirm?

Can you also tell if IBRS must be written as a barrier to SPEC_CTRL in
return to userland (kernel exit) when ibrs_enabled 2? Generally we
wouldn't run a barrier there with ibrs_enabled 2, but absolutely
nothing is intuitive here so I need to ask explicitly.