sparc64: Fix register corruption in top-most kernel stack frame during boot.

Meelis Roos reported that kernels built with gcc-4.9 do not boot, we
eventually narrowed this down to only impacting machines using
UltraSPARC-III and derivitive cpus.

The crash happens right when the first user process is spawned:

[   54.451346] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
[   54.451346]
[   54.571516] CPU: 1 PID: 1 Comm: init Not tainted 3.16.0-rc2-00211-gd7933ab #96
[   54.666431] Call Trace:
[   54.698453]  [0000000000762f8c] panic+0xb0/0x224
[   54.759071]  [000000000045cf68] do_exit+0x948/0x960
[   54.823123]  [000000000042cbc0] fault_in_user_windows+0xe0/0x100
[   54.902036]  [0000000000404ad0] __handle_user_windows+0x0/0x10
[   54.978662] Press Stop-A (L1-A) to return to the boot prom
[   55.050713] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004

Further investigation showed that compiling only per_cpu_patch() with
an older compiler fixes the boot.

Detailed analysis showed that the function is not being miscompiled by
gcc-4.9, but it is using a different register allocation ordering.

With the gcc-4.9 compiled function, something during the code patching
causes some of the %i* input registers to get corrupted.  Perhaps
we have a TLB miss path into the firmware that is deep enough to
cause a register window spill and subsequent restore when we get
back from the TLB miss trap.

Let's plug this up by doing two things:

1) Stop using the firmware stack for client interface calls into
   the firmware.  Just use the kernel's stack.

2) As soon as we can, call into a new function "start_early_boot()"
   to put a one-register-window buffer between the firmware's
   deepest stack frame and the top-most initial kernel one.

Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 36 deletions

diff --git a/arch/sparc/kernel/head_64.S b/arch/sparc/kernel/head_64.S
index 4fdeb8040d4d..3d61fcae7ee3 100644
--- a/arch/sparc/kernel/head_64.S
+++ b/arch/sparc/kernel/head_64.S
@@ -672,14 +672,12 @@ tlb_fixup_done:
 	sethi	%hi(init_thread_union), %g6
 	or	%g6, %lo(init_thread_union), %g6
 	ldx	[%g6 + TI_TASK], %g4
-	mov	%sp, %l6
 
 	wr	%g0, ASI_P, %asi
 	mov	1, %g1
 	sllx	%g1, THREAD_SHIFT, %g1
 	sub	%g1, (STACKFRAME_SZ + STACK_BIAS), %g1
 	add	%g6, %g1, %sp
-	mov	0, %fp
 
 	/* Set per-cpu pointer initially to zero, this makes
 	 * the boot-cpu use the in-kernel-image per-cpu areas
@@ -706,44 +704,14 @@ tlb_fixup_done:
 	 nop
 #endif
 
-	mov	%l6, %o1			! OpenPROM stack
 	call	prom_init
 	 mov	%l7, %o0			! OpenPROM cif handler
 
-	/* Initialize current_thread_info()->cpu as early as possible.
-	 * In order to do that accurately we have to patch up the get_cpuid()
-	 * assembler sequences.  And that, in turn, requires that we know
-	 * if we are on a Starfire box or not.  While we're here, patch up
-	 * the sun4v sequences as well.
+	/* To create a one-register-window buffer between the kernel's
+	 * initial stack and the last stack frame we use from the firmware,
+	 * do the rest of the boot from a C helper function.
 	 */
-	call	check_if_starfire
-	 nop
-	call	per_cpu_patch
-	 nop
-	call	sun4v_patch
-	 nop
-
-#ifdef CONFIG_SMP
-	call	hard_smp_processor_id
-	 nop
-	cmp	%o0, NR_CPUS
-	blu,pt	%xcc, 1f
-	 nop
-	call	boot_cpu_id_too_large
-	 nop
-	/* Not reached... */
-
-1:
-#else
-	mov	0, %o0
-#endif
-	sth	%o0, [%g6 + TI_CPU]
-
-	call	prom_init_report
-	 nop
-
-	/* Off we go.... */
-	call	start_kernel
+	call	start_early_boot
 	 nop
 	/* Not reached... */