[PATCH v2 1/2] powerpc/64: Set up a kernel stack for secondaries before cpu_restore()

[Jordan Niethe]

Currently in generic_secondary_smp_init(), cur_cpu_spec->cpu_restore()
is called before a stack has been set up in r1. This was previously fine
as the cpu_restore() functions were implemented in assembly and did not
use a stack. However commit 5a61ef74f269 ("powerpc/64s: Support new
device tree binding for discovering CPU features") used
__restore_cpu_cpufeatures() as the cpu_restore() function for a
device-tree features based cputable entry. This is a C function and
hence uses a stack in r1.

generic_secondary_smp_init() is entered on the secondary cpus via the
primary cpu using the OPAL call opal_start_cpu(). In OPAL, each hardware
thread has its own stack. The OPAL call is ran in the primary's hardware
thread. During the call, a job is scheduled on a secondary cpu that will
start executing at the address of generic_secondary_smp_init().  Hence
the value that will be left in r1 when the secondary cpu enters the
kernel is part of that secondary cpu's individual OPAL stack. This means
that __restore_cpu_cpufeatures() will write to that OPAL stack. This is
not horribly bad as each hardware thread has its own stack and the call
that enters the kernel from OPAL never returns, but it is still wrong
and should be corrected.

Create the temp kernel stack before calling cpu_restore().

Fixes: 5a61ef74f269 ("powerpc/64s: Support new device tree binding for discovering CPU features")
Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
---
v2: Add more detail to the commit message
---
 arch/powerpc/kernel/head_64.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 0e05a9a47a4b..4b7f4c6c2600 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -420,6 +420,10 @@ generic_secondary_common_init:
 	/* From now on, r24 is expected to be logical cpuid */
 	mr	r24,r5
 
+	/* Create a temp kernel stack for use before relocation is on.	*/
+	ld	r1,PACAEMERGSP(r13)
+	subi	r1,r1,STACK_FRAME_OVERHEAD
+
 	/* See if we need to call a cpu state restore handler */
 	LOAD_REG_ADDR(r23, cur_cpu_spec)
 	ld	r23,0(r23)
@@ -448,10 +452,6 @@ generic_secondary_common_init:
 	sync				/* order paca.run and cur_cpu_spec */
 	isync				/* In case code patching happened */
 
-	/* Create a temp kernel stack for use before relocation is on.	*/
-	ld	r1,PACAEMERGSP(r13)
-	subi	r1,r1,STACK_FRAME_OVERHEAD
-
 	b	__secondary_start
 #endif /* SMP */
 
-- 
2.17.1