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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@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

FWIW, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, 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@xxxxxxxxxx>
Date: Mon, 18 Apr 2022 09:50:25 -0700

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

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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