linux-kernel - Re: [PATCH 14/29] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Tue, 22 Feb 2022 04:38:22 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Poimboe, Josh" <jpoimboe@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "Milburn, Alyssa" <alyssa.milburn@...el.com>
Subject: Re: [PATCH 14/29] x86/ibt: Add IBT feature, MSR and #CP handling

On Fri, 2022-02-18 at 17:49 +0100, Peter Zijlstra wrote:
> +       cr4_set_bits(X86_CR4_CET);
> +
> +       rdmsrl(MSR_IA32_S_CET, msr);
> +       if (cpu_feature_enabled(X86_FEATURE_IBT))
> +               msr |= CET_ENDBR_EN;
> +       wrmsrl(MSR_IA32_S_CET, msr);

So I guess implicit in all of this is that MSR_IA32_S_CET will not be
managed by xsaves (makes sense).

But it still might be good to add the supervisor cet xfeature number to
XFEATURE_MASK_SUPERVISOR_UNSUPPORTED, with analogous reasoning to
XFEATURE_MASK_PT.