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

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Message-ID: <b94cbac6-0a4d-8e4a-ec58-bbd46e385d45@citrix.com>
Date:   Tue, 19 Apr 2022 11:42:12 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.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 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.

~Andrew