linux-kernel - Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on Enhanced IBRS parts

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Message-ID: <20220714173814.p5kdyimu6ho7zjt5@treble>
Date:   Thu, 14 Jul 2022 10:38:14 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on
 Enhanced IBRS parts

On Thu, Jul 14, 2022 at 07:03:32PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 14, 2022 at 09:01:06AM -0700, Josh Poimboeuf wrote:
> 
> > > Yeah this; if the user asks for IBRS, we should give him IBRS. I hate
> > > the 'I know better, let me change that for you' mentality.
> > 
> > eIBRS CPUs don't even have legacy IBRS so I don't see how this is even
> > possible.
> 
> You can still WRMSR a lot on them. Might not make sense but it 'works'.

Even in Intel documentation, eIBRS is often referred to as IBRS. It
wouldn't be surprising for a user to consider spectre_v2=ibrs to mean
"use eIBRS".

I'm pretty sure there's nobody out there that wants spectre_v2=ibrs to
mean "make it slower and possibly less secure because it's being used
contrary to the spec".

-- 
Josh