MIPS: kernel: Reserve exception base early to prevent corruption

BMIPS is one of the few platforms that do change the exception base.
After commit 2dcb3964 ("memblock: do not start bottom-up allocations
with kernel_end") we started seeing BMIPS boards fail to boot with the
built-in FDT being corrupted.

Before the cited commit, early allocations would be in the [kernel_end,
RAM_END] range, but after commit they would be within [RAM_START +
PAGE_SIZE, RAM_END].

The custom exception base handler that is installed by
bmips_ebase_setup() done for BMIPS5000 CPUs ends-up trampling on the
memory region allocated by unflatten_and_copy_device_tree() thus
corrupting the FDT used by the kernel.

To fix this, we need to perform an early reservation of the custom
exception space. Additional we reserve the first 4k (1k for R3k) for
either normal exception vector space (legacy CPUs) or special vectors
like cache exceptions.

Huge thanks to Serge for analysing and proposing a solution to this
issue.

Fixes: 2dcb3964 ("memblock: do not start bottom-up allocations with kernel_end")
Reported-by: Kamal Dasu <kdasu.kdev@gmail.com>
Debugged-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/traps.h

@@ -24,8 +24,11 @@ extern void (*board_ebase_setup)(void);
 extern void (*board_cache_error_setup)(void);
 
 extern int register_nmi_notifier(struct notifier_block *nb);
+extern void reserve_exception_space(phys_addr_t addr, unsigned long size);
 extern char except_vec_nmi[];
 
+#define VECTORSPACING 0x100	/* for EI/VI mode */
+
 #define nmi_notifier(fn, pri)						\
 ({									\
 	static struct notifier_block fn##_nb = {			\

arch/mips/kernel/cpu-probe.c

@@ -26,6 +26,7 @@
 #include <asm/elf.h>
 #include <asm/pgtable-bits.h>
 #include <asm/spram.h>
+#include <asm/traps.h>
 #include <linux/uaccess.h>
 
 #include "fpu-probe.h"
@@ -1628,6 +1629,7 @@ static inline void cpu_probe_broadcom(struct cpuinfo_mips *c, unsigned int cpu)
 		c->cputype = CPU_BMIPS3300;
 		__cpu_name[cpu] = "Broadcom BMIPS3300";
 		set_elf_platform(cpu, "bmips3300");
+		reserve_exception_space(0x400, VECTORSPACING * 64);
 		break;
 	case PRID_IMP_BMIPS43XX: {
 		int rev = c->processor_id & PRID_REV_MASK;
@@ -1638,6 +1640,7 @@ static inline void cpu_probe_broadcom(struct cpuinfo_mips *c, unsigned int cpu)
 			__cpu_name[cpu] = "Broadcom BMIPS4380";
 			set_elf_platform(cpu, "bmips4380");
 			c->options |= MIPS_CPU_RIXI;
+			reserve_exception_space(0x400, VECTORSPACING * 64);
 		} else {
 			c->cputype = CPU_BMIPS4350;
 			__cpu_name[cpu] = "Broadcom BMIPS4350";
@@ -1654,6 +1657,7 @@ static inline void cpu_probe_broadcom(struct cpuinfo_mips *c, unsigned int cpu)
 			__cpu_name[cpu] = "Broadcom BMIPS5000";
 		set_elf_platform(cpu, "bmips5000");
 		c->options |= MIPS_CPU_ULRI | MIPS_CPU_RIXI;
+		reserve_exception_space(0x1000, VECTORSPACING * 64);
 		break;
 	}
 }
@@ -2133,6 +2137,8 @@ void cpu_probe(void)
 	if (cpu == 0)
 		__ua_limit = ~((1ull << cpu_vmbits) - 1);
 #endif
+
+	reserve_exception_space(0, 0x1000);
 }
 
 void cpu_report(void)

arch/mips/kernel/cpu-r3k-probe.c

@@ -21,6 +21,7 @@
 #include <asm/fpu.h>
 #include <asm/mipsregs.h>
 #include <asm/elf.h>
+#include <asm/traps.h>
 
 #include "fpu-probe.h"
 
@@ -158,6 +159,8 @@ void cpu_probe(void)
 		cpu_set_fpu_opts(c);
 	else
 		cpu_set_nofpu_opts(c);
+
+	reserve_exception_space(0, 0x400);
 }
 
 void cpu_report(void)

arch/mips/kernel/traps.c

@@ -2009,13 +2009,16 @@ void __noreturn nmi_exception_handler(struct pt_regs *regs)
 	nmi_exit();
 }
 
-#define VECTORSPACING 0x100	/* for EI/VI mode */
-
 unsigned long ebase;
 EXPORT_SYMBOL_GPL(ebase);
 unsigned long exception_handlers[32];
 unsigned long vi_handlers[64];
 
+void reserve_exception_space(phys_addr_t addr, unsigned long size)
+{
+	memblock_reserve(addr, size);
+}
+
 void __init *set_except_vector(int n, void *addr)
 {
 	unsigned long handler = (unsigned long) addr;
@@ -2367,10 +2370,7 @@ void __init trap_init(void)
 
 	if (!cpu_has_mips_r2_r6) {
 		ebase = CAC_BASE;
-		ebase_pa = virt_to_phys((void *)ebase);
 		vec_size = 0x400;
-
-		memblock_reserve(ebase_pa, vec_size);
 	} else {
 		if (cpu_has_veic || cpu_has_vint)
 			vec_size = 0x200 + VECTORSPACING*64;