RISC-V: Always compile mm/init.c with cmodel=medany and notrace

The Linux RISC-V 32bit kernel is broken after we moved setup_vm() from
kernel/setup.c to mm/init.c because Linux RISC-V 32bit kernel by default
uses cmodel=medlow which results in a non-position-independent setup_vm().

This patch fixes Linux RISC-V 32bit kernel booting by:
1. Forcing cmodel=medany for mm/init.c
2. Moving remaing MM-related stuff va_pa_offset, pfn_base and
   empty_zero_page from kernel/setup.c to mm/init.c

Further, the setup_vm() cannot handle GCC instrumentation for FTRACE so
we disable it for mm/init.c by not using "-pg" compiler flag.

Fixes: 6f1e9e946f0b ("RISC-V: Move setup_vm() to mm/init.c")
Suggested-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

2 files changed, 34 insertions, 0 deletions

diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
index eb22ab49b3e0..b68aac701803 100644
--- a/arch/riscv/mm/Makefile
+++ b/arch/riscv/mm/Makefile
@@ -1,3 +1,9 @@
+
+CFLAGS_init.o := -mcmodel=medany
+ifdef CONFIG_FTRACE
+CFLAGS_REMOVE_init.o = -pg
+endif
+
 obj-y += init.o
 obj-y += fault.o
 obj-y += extable.o
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index b379a75ac6a6..5fd8c922e1c2 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -25,6 +25,10 @@
 #include <asm/pgtable.h>
 #include <asm/io.h>
 
+unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
+							__page_aligned_bss;
+EXPORT_SYMBOL(empty_zero_page);
+
 static void __init zone_sizes_init(void)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
@@ -143,6 +147,11 @@ void __init setup_bootmem(void)
 	}
 }
 
+unsigned long va_pa_offset;
+EXPORT_SYMBOL(va_pa_offset);
+unsigned long pfn_base;
+EXPORT_SYMBOL(pfn_base);
+
 pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
 pgd_t trampoline_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
 
@@ -172,6 +181,25 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 	}
 }
 
+/*
+ * setup_vm() is called from head.S with MMU-off.
+ *
+ * Following requirements should be honoured for setup_vm() to work
+ * correctly:
+ * 1) It should use PC-relative addressing for accessing kernel symbols.
+ *    To achieve this we always use GCC cmodel=medany.
+ * 2) The compiler instrumentation for FTRACE will not work for setup_vm()
+ *    so disable compiler instrumentation when FTRACE is enabled.
+ *
+ * Currently, the above requirements are honoured by using custom CFLAGS
+ * for init.o in mm/Makefile.
+ */
+
+#ifndef __riscv_cmodel_medany
+#error "setup_vm() is called from head.S before relocate so it should "
+	"not use absolute addressing."
+#endif
+
 asmlinkage void __init setup_vm(void)
 {
 	extern char _start;