linux-kernel - Re: [PATCH v9 7/8] x86/cpu: Support AMD Automatic IBRS

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Date:   Fri, 24 Feb 2023 23:51:17 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Kim Phillips <kim.phillips@....com>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Alexey Kardashevskiy <aik@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 7/8] x86/cpu: Support AMD Automatic IBRS

On Fri, Feb 24, 2023 at 10:08:32PM +0100, Borislav Petkov wrote:
> On Fri, Feb 24, 2023 at 10:52:57AM -0800, Josh Poimboeuf wrote:
> > Doesn't this only enable it on the boot CPU?
> 
> Whoops, you might be right.

Actually, we stick that MSR - EFER - into the trampoline header and then
each AP gets it written to in arch/x86/realmode/rm/trampoline_64.S

But this is only from code staring - I'll confirm this tomorrow.

And if so, we should at least put comments in that trampoline code so
that people do not remove the MSR writes.

Or, actually, we should simply write it again because it is the init
path and not really a hot path but it should damn well make sure that
that bit gets set.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette