linux-kernel - Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Fri, 25 Feb 2022 12:10:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Fri, Feb 25, 2022 at 11:51:01AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 24, 2022 at 03:55:16PM -0800, Josh Poimboeuf wrote:
> > On Thu, Feb 24, 2022 at 03:51:56PM +0100, Peter Zijlstra wrote:
> > > +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> > > +{
> > > +	u64 msr = CET_ENDBR_EN;
> > > +
> > > +	if (!HAS_KERNEL_IBT ||
> > > +	    !cpu_feature_enabled(X86_FEATURE_IBT))
> > > +		return;
> > 
> > If you add X86_FEATURE_BIT to arch/x86/include/asm/disabled-features.h,
> > the HAS_KERNEL_IBT check becomes redundant.
> 
> Cute.

On second thought; I'm not sure that's desirable. Ideally KVM would
still expose IBT if present on the hardware, even if the host kernel
doesn't use it.

Killing the feature when the host doesn't use it seems unfortunate.