linux-kernel - Re: [PATCH v2 06/25] x86/xen: Add ANNOTATE_ENDBR to startup

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Date:   Tue, 19 Apr 2022 13:57:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH v2 06/25] x86/xen: Add ANNOTATE_ENDBR to startup_xen()

On Tue, Apr 19, 2022 at 11:42:12AM +0000, Andrew Cooper wrote:
> On 18/04/2022 17:50, Josh Poimboeuf wrote:
> > The startup_xen() kernel entry point is referenced by the ".note.Xen"
> > section, but is presumably not indirect-branched to.
> 
> It's the real entrypoint of the VM.  It's "got to" by setting %rip
> during vcpu setup.
> 
> We could in principle support starting a PV VM with CET active, but that
> sounds like an enormous quantity of effort for very little gain.  CET
> for Xen PV requires paravirt anyway (because the kernel runs in CPL!=0)
> so decisions like this can wait until someone feels like doing the work.
> 
> >   Add ANNOTATE_ENDBR
> > to silence future objtool warnings.
> >
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Juergen Gross <jgross@...e.com>
> > Cc: Stefano Stabellini <sstabellini@...nel.org>
> > Cc: xen-devel@...ts.xenproject.org
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> 
> FWIW, Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>, preferably
> with the commit message tweaked to remove the uncertainty.

Something like so then?

---
Subject: x86/xen: Add ANNOTATE_ENDBR to startup_xen()
From: Josh Poimboeuf <jpoimboe@...hat.com>
Date: Mon, 18 Apr 2022 09:50:25 -0700

From: Josh Poimboeuf <jpoimboe@...hat.com>

The startup_xen() kernel entry point is referenced by the ".note.Xen"
section, and is the real entry point of the VM. It *will* be
indirectly branched to, *however* currently Xen doesn't support PV VM
with CET active.

Add ANNOTATE_ENDBR to silence future objtool warnings.

Fixes: ed53a0d97192 ("x86/alternative: Use .ibt_endbr_seal to seal indirect calls")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>
Link: https://lkml.kernel.org/r/a87bd48b06d11ec4b98122a429e71e489b4e48c3.1650300597.git.jpoimboe@redhat.com