powerpc/64s: early boot machine check handler

Use the early boot interrupt fixup in the machine check handler to allow
the machine check handler to run before interrupt endian is set up.
Branch to an early boot handler that just does a basic crash, which
allows it to run before ppc_md is set up. MSR[ME] is enabled on the boot
CPU earlier, and the machine check stack is temporarily set to the
middle of the init task stack.

This allows machine checks (e.g., due to invalid data access in real
mode) to print something useful earlier in boot (as soon as udbg is set
up, if CONFIG_PPC_EARLY_DEBUG=y).
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926055620.2676869-3-npiggin@gmail.com

arch/powerpc/include/asm/asm-prototypes.h

@@ -36,6 +36,7 @@ int64_t __opal_call(int64_t a0, int64_t a1, int64_t a2, int64_t a3,
 		    int64_t opcode, uint64_t msr);
 
 /* misc runtime */
+void enable_machine_check(void);
 extern u64 __bswapdi2(u64);
 extern s64 __lshrdi3(s64, int);
 extern s64 __ashldi3(s64, int);

arch/powerpc/kernel/exceptions-64s.S

@@ -1134,6 +1134,7 @@ INT_DEFINE_BEGIN(machine_check)
 INT_DEFINE_END(machine_check)
 
 EXC_REAL_BEGIN(machine_check, 0x200, 0x100)
+	EARLY_BOOT_FIXUP
 	GEN_INT_ENTRY machine_check_early, virt=0
 EXC_REAL_END(machine_check, 0x200, 0x100)
 EXC_VIRT_NONE(0x4200, 0x100)
@@ -1198,6 +1199,9 @@ BEGIN_FTR_SECTION
 	bl	enable_machine_check
 END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
 	addi	r3,r1,STACK_FRAME_OVERHEAD
+BEGIN_FTR_SECTION
+	bl	machine_check_early_boot
+END_FTR_SECTION(0, 1)     // nop out after boot
 	bl	machine_check_early
 	std	r3,RESULT(r1)	/* Save result */
 	ld	r12,_MSR(r1)
@@ -3096,7 +3100,7 @@ CLOSE_FIXED_SECTION(virt_trampolines);
 USE_TEXT_SECTION()
 
 /* MSR[RI] should be clear because this uses SRR[01] */
-enable_machine_check:
+_GLOBAL(enable_machine_check)
 	mflr	r0
 	bcl	20,31,$+4
 0:	mflr	r3

arch/powerpc/kernel/setup_64.c

@@ -34,6 +34,7 @@
 #include <linux/of.h>
 #include <linux/of_fdt.h>
 
+#include <asm/asm-prototypes.h>
 #include <asm/kvm_guest.h>
 #include <asm/io.h>
 #include <asm/kdump.h>
@@ -180,6 +181,16 @@ static void __init fixup_boot_paca(void)
 {
 	/* The boot cpu is started */
 	get_paca()->cpu_start = 1;
+#ifdef CONFIG_PPC_BOOK3S_64
+	/*
+	 * Give the early boot machine check stack somewhere to use, use
+	 * half of the init stack. This is a bit hacky but there should not be
+	 * deep stack usage in early init so shouldn't overflow it or overwrite
+	 * things.
+	 */
+	get_paca()->mc_emergency_sp = (void *)&init_thread_union +
+		(THREAD_SIZE/2);
+#endif
 	/* Allow percpu accesses to work until we setup percpu data */
 	get_paca()->data_offset = 0;
 	/* Mark interrupts soft and hard disabled in PACA */
@@ -357,6 +368,9 @@ void __init early_setup(unsigned long dt_ptr)
 
 	/* -------- printk is now safe to use ------- */
 
+	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && (mfmsr() & MSR_HV))
+		enable_machine_check();
+
 	/* Try new device tree based feature discovery ... */
 	if (!dt_cpu_ftrs_init(__va(dt_ptr)))
 		/* Otherwise use the old style CPU table */

arch/powerpc/kernel/traps.c

@@ -68,6 +68,7 @@
 #include <asm/stacktrace.h>
 #include <asm/nmi.h>
 #include <asm/disassemble.h>
+#include <asm/udbg.h>
 
 #if defined(CONFIG_DEBUGGER) || defined(CONFIG_KEXEC_CORE)
 int (*__debugger)(struct pt_regs *regs) __read_mostly;
@@ -850,6 +851,19 @@ static void __machine_check_exception(struct pt_regs *regs)
 }
 
 #ifdef CONFIG_PPC_BOOK3S_64
+DEFINE_INTERRUPT_HANDLER_RAW(machine_check_early_boot)
+{
+	udbg_printf("Machine check (early boot)\n");
+	udbg_printf("SRR0=0x%016lx   SRR1=0x%016lx\n", regs->nip, regs->msr);
+	udbg_printf(" DAR=0x%016lx  DSISR=0x%08lx\n", regs->dar, regs->dsisr);
+	udbg_printf("  LR=0x%016lx     R1=0x%08lx\n", regs->link, regs->gpr[1]);
+	udbg_printf("------\n");
+	die("Machine check (early boot)", regs, SIGBUS);
+	for (;;)
+		;
+	return 0;
+}
+
 DEFINE_INTERRUPT_HANDLER_ASYNC(machine_check_exception_async)
 {
 	__machine_check_exception(regs);