xen/p2m: An early bootup variant of set_phys_to_machine

During early bootup we can't use alloc_page, so to allocate
leaf pages in the P2M we need to use extend_brk. For that
we are utilizing the early_alloc_p2m and early_alloc_p2m_middle
functions to do the job for us. This function follows the
same logic as set_phys_to_machine.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/include/asm/xen/page.h

@@ -44,6 +44,7 @@ extern unsigned long  machine_to_phys_nr;
 
 extern unsigned long get_phys_to_machine(unsigned long pfn);
 extern bool set_phys_to_machine(unsigned long pfn, unsigned long mfn);
+extern bool __init early_set_phys_to_machine(unsigned long pfn, unsigned long mfn);
 extern bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn);
 extern unsigned long set_phys_range_identity(unsigned long pfn_s,
 					     unsigned long pfn_e);

arch/x86/xen/p2m.c

@@ -570,6 +570,21 @@ static bool __init early_alloc_p2m(unsigned long pfn)
 	}
 	return true;
 }
+bool __init early_set_phys_to_machine(unsigned long pfn, unsigned long mfn)
+{
+	if (unlikely(!__set_phys_to_machine(pfn, mfn)))  {
+		if (!early_alloc_p2m(pfn))
+			return false;
+
+		if (!early_alloc_p2m_middle(pfn, false /* boundary crossover OK!*/))
+			return false;
+
+		if (!__set_phys_to_machine(pfn, mfn))
+			return false;
+	}
+
+	return true;
+}
 unsigned long __init set_phys_range_identity(unsigned long pfn_s,
 				      unsigned long pfn_e)
 {