linux-kernel - Re: [PATCH 0/7] IBRS patch series

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Message-ID: <20180105164245.GW26807@redhat.com>
Date:   Fri, 5 Jan 2018 17:42:45 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/7] IBRS patch series

On Fri, Jan 05, 2018 at 04:37:30PM +0000, David Woodhouse wrote:
> You are completely ignoring pre-Skylake here.
> 
> On pre-Skylake, retpoline is perfectly sufficient and it's a *lot*
> faster than the IBRS option which is almost prohibitively slow.
> 
> We didn't do it just for fun. And it's working fine; it isn't *that*
> complex.

How do you enable IBRS when the CPU switches to SMM?

Do you already have this 2-way code emission from gcc and patching
with a 3-way alternatives at boot between ibrs and 2 reptoline version
emitted by gcc and alternatives between ibrs and ibpb where SPEC_CTRL
is missing on some CPU but IBPB_SUPPORT is available?

Or are you talking about having done this on a non upstream Xen build
only without the 2-way code emission for gcc?