x86/xen: Make the boot CPU idle task reliable
The unwinder reports the boot CPU idle task's stack on XEN PV as unreliable, which affects at least live patching. There are two reasons for this. First, the task does not follow the x86 convention that its stack starts at the offset right below saved pt_regs. It allows the unwinder to easily detect the end of the stack and verify it. Second, startup_xen() function does not store the return address before jumping to xen_start_kernel() which confuses the unwinder. Amend both issues by moving the starting point of initial stack in startup_xen() and storing the return address before the jump, which is exactly what call instruction does. Signed-off-by: Miroslav Benes <mbenes@suse.cz> Reviewed-by: Juergen Gross <jgross@suse.com> Signed-off-by: Juergen Gross <jgross@suse.com>
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