linux-kernel - Build warnings in Xen 5.15.y and 5.10.y with retbleed backports

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Date:   Tue, 12 Jul 2022 18:38:44 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Subject: Build warnings in Xen 5.15.y and 5.10.y with retbleed backports

Hi all,

I'm seeing the following build warning:
	arch/x86/kernel/head_64.o: warning: objtool: xen_hypercall_mmu_update(): can't find starting instruction
in the 5.15.y and 5.10.y retbleed backports.

I don't know why just this one hypercall is being called out by objtool,
and this warning isn't in 5.18 and Linus's tree due to I think commit
5b2fc51576ef ("x86/ibt,xen: Sprinkle the ENDBR") being there.

But, is this a ret call that we "forgot" here?  It's a "real" ret in
Linus's branch:

.pushsection .noinstr.text, "ax"
	.balign PAGE_SIZE
SYM_CODE_START(hypercall_page)
	.rept (PAGE_SIZE / 32)
		UNWIND_HINT_FUNC
		ANNOTATE_NOENDBR
		ANNOTATE_UNRET_SAFE
		ret
		/*
		 * Xen will write the hypercall page, and sort out ENDBR.
		 */
		.skip 31, 0xcc
	.endr

while 5.15.y and older has:
.pushsection .text
	.balign PAGE_SIZE
SYM_CODE_START(hypercall_page)
	.rept (PAGE_SIZE / 32)
		UNWIND_HINT_FUNC
		.skip 31, 0x90
		ANNOTATE_UNRET_SAFE
		RET
	.endr

So should the "ret" remain or be turned into "RET" in mainline right
now?

thanks,

greg k-h