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

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Message-ID: <e1e11062-f528-ea06-cb94-fd4dadf2c5aa@suse.com>
Date:   Tue, 19 Apr 2022 14:06:18 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 19.04.22 13:57, Peter Zijlstra wrote:
> 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.

Hmm, Xen will always use IRET to activate the guest.


Juergen

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