s390/mm: allocate Real Memory Copy Area in decompressor

Move Real Memory Copy Area allocation to the decompressor.
As result, memcpy_real() and memcpy_real_iter() movers
become usable since the very moment the kernel starts.

Reviewed-by: Heiko Carstens <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/boot/startup.c

···
 #include <linux/elf.h>
 #include <asm/boot_data.h>
 #include <asm/sections.h>
+#include <asm/maccess.h>
 #include <asm/cpu_mf.h>
 #include <asm/setup.h>
 #include <asm/kasan.h>
···
 unsigned long __bootdata_preserved(__kaslr_offset);
 unsigned long __bootdata_preserved(__abs_lowcore);
 unsigned long __bootdata_preserved(__memcpy_real_area);
+pte_t *__bootdata_preserved(memcpy_real_ptep);
 unsigned long __bootdata(__amode31_base);
 unsigned long __bootdata_preserved(VMALLOC_START);
 unsigned long __bootdata_preserved(VMALLOC_END);

arch/s390/boot/vmem.c

···
 #include <asm/facility.h>
 #include <asm/sections.h>
 #include <asm/mem_detect.h>
+#include <asm/maccess.h>
 #include "decompressor.h"
 #include "boot.h"
 
···
 #define swapper_pg_dir		vmlinux.swapper_pg_dir_off
 #define invalid_pg_dir		vmlinux.invalid_pg_dir_off
 
+/*
+ * Mimic virt_to_kpte() in lack of init_mm symbol. Skip pmd NULL check though.
+ */
+static inline pte_t *__virt_to_kpte(unsigned long va)
+{
+	return pte_offset_kernel(pmd_offset(pud_offset(p4d_offset(pgd_offset_k(va), va), va), va), va);
+}
+
 unsigned long __bootdata_preserved(s390_invalid_asce);
 unsigned long __bootdata(pgalloc_pos);
 unsigned long __bootdata(pgalloc_end);
 unsigned long __bootdata(pgalloc_low);
 
 enum populate_mode {
+	POPULATE_NONE,
 	POPULATE_ONE2ONE,
 };
 
···
 static unsigned long _pa(unsigned long addr, enum populate_mode mode)
 {
 	switch (mode) {
+	case POPULATE_NONE:
+		return -1;
 	case POPULATE_ONE2ONE:
 		return addr;
 	default:
···
 	pgtable_populate_begin(online_end);
 	pgtable_populate(0, sizeof(struct lowcore), POPULATE_ONE2ONE);
 	pgtable_populate(0, online_end, POPULATE_ONE2ONE);
+	pgtable_populate(__memcpy_real_area, __memcpy_real_area + PAGE_SIZE,
+			 POPULATE_NONE);
+	memcpy_real_ptep = __virt_to_kpte(__memcpy_real_area);
 	pgtable_populate_end();
 
 	S390_lowcore.kernel_asce = swapper_pg_dir | asce_bits;

arch/s390/include/asm/maccess.h

···
 struct iov_iter;
 
 extern unsigned long __memcpy_real_area;
-void memcpy_real_init(void);
+extern pte_t *memcpy_real_ptep;
 size_t memcpy_real_iter(struct iov_iter *iter, unsigned long src, size_t count);
 int memcpy_real(void *dest, unsigned long src, size_t count);
 #ifdef CONFIG_CRASH_DUMP

arch/s390/kernel/setup.c

···
 	 * Create kernel page tables.
 	 */
         paging_init();
-	memcpy_real_init();
+
 	/*
 	 * After paging_init created the kernel page table, the new PSWs
 	 * in lowcore can now run with DAT enabled.

arch/s390/mm/maccess.c

···
 #include <asm/maccess.h>
 
 unsigned long __bootdata_preserved(__memcpy_real_area);
-static __ro_after_init pte_t *memcpy_real_ptep;
+pte_t *__bootdata_preserved(memcpy_real_ptep);
 static DEFINE_MUTEX(memcpy_real_mutex);
 
 static notrace long s390_kernel_write_odd(void *dst, const void *src, size_t size)
···
 	spin_unlock_irqrestore(&s390_kernel_write_lock, flags);
 
 	return dst;
-}
-
-void __init memcpy_real_init(void)
-{
-	memcpy_real_ptep = vmem_get_alloc_pte(__memcpy_real_area, true);
-	if (!memcpy_real_ptep)
-		panic("Couldn't setup memcpy real area");
 }
 
 size_t memcpy_real_iter(struct iov_iter *iter, unsigned long src, size_t count)