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

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Date:   Fri, 24 Feb 2023 16:09:31 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
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 11:51:17PM +0100, Borislav Petkov wrote:
> 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.

Ah, I had to stare it that for a bit to figure out how it works.
setup_real_mode() reads MSR_EFER from the boot CPU and stores it in
trampoline_header->efer.  Then the other CPUs read that stored value in
startup_32() and write it into their MSR.

> 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.

Yeah, I think that would be good.  Otherwise it's rather magical.  That
EFER MSR is a surprising place to put that bit.

-- 
Josh