mm: provide early_memremap_ro to establish read-only mapping

During early boot as Xen pv domain the kernel needs to map some page
tables supplied by the hypervisor read only. This is needed to be
able to relocate some data structures conflicting with the physical
memory map especially on systems with huge RAM (above 512GB).

Provide the function early_memremap_ro() to provide this read only
mapping.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

include/asm-generic/early_ioremap.h

@@ -11,6 +11,8 @@ extern void __iomem *early_ioremap(resource_size_t phys_addr,
 				   unsigned long size);
 extern void *early_memremap(resource_size_t phys_addr,
 			    unsigned long size);
+extern void *early_memremap_ro(resource_size_t phys_addr,
+			       unsigned long size);
 extern void early_iounmap(void __iomem *addr, unsigned long size);
 extern void early_memunmap(void *addr, unsigned long size);
 

include/asm-generic/fixmap.h

@@ -46,6 +46,9 @@ static inline unsigned long virt_to_fix(const unsigned long vaddr)
 #ifndef FIXMAP_PAGE_NORMAL
 #define FIXMAP_PAGE_NORMAL PAGE_KERNEL
 #endif
+#if !defined(FIXMAP_PAGE_RO) && defined(PAGE_KERNEL_RO)
+#define FIXMAP_PAGE_RO PAGE_KERNEL_RO
+#endif
 #ifndef FIXMAP_PAGE_NOCACHE
 #define FIXMAP_PAGE_NOCACHE PAGE_KERNEL_NOCACHE
 #endif

mm/early_ioremap.c

@@ -217,6 +217,13 @@ early_memremap(resource_size_t phys_addr, unsigned long size)
 	return (__force void *)__early_ioremap(phys_addr, size,
 					       FIXMAP_PAGE_NORMAL);
 }
+#ifdef FIXMAP_PAGE_RO
+void __init *
+early_memremap_ro(resource_size_t phys_addr, unsigned long size)
+{
+	return (__force void *)__early_ioremap(phys_addr, size, FIXMAP_PAGE_RO);
+}
+#endif
 #else /* CONFIG_MMU */
 
 void __init __iomem *
@@ -231,6 +238,11 @@ early_memremap(resource_size_t phys_addr, unsigned long size)
 {
 	return (void *)phys_addr;
 }
+void __init *
+early_memremap_ro(resource_size_t phys_addr, unsigned long size)
+{
+	return (void *)phys_addr;
+}
 
 void __init early_iounmap(void __iomem *addr, unsigned long size)
 {