[PATCH] arm64: Minor refactoring of cpu_switch_to() to fix build breakage

[Ingo Molnar]

* Olof Johansson <olof at lixom.net> wrote:

> Commit 0c8c0f03e3a2 ("x86/fpu, sched: Dynamically allocate 'struct fpu'")
> moved the thread_struct to the bottom of task_struct. As a result, the
> offset is now too large to be used in an immediate add on arm64 with
> some kernel configs:
> 
> arch/arm64/kernel/entry.S: Assembler messages:
> arch/arm64/kernel/entry.S:588: Error: immediate out of range
> arch/arm64/kernel/entry.S:597: Error: immediate out of range
> 
> There's really no reason for cpu_switch_to to take a task_struct pointer
> in the first place, since all it does is access the thread.cpu_context
> member. So, just pass that in directly.
> 
> Fixes: 0c8c0f03e3a2 ("x86/fpu, sched: Dynamically allocate 'struct fpu'")
> Cc: Dave Hansen <dave.hansen at linux.intel.com>
> Signed-off-by: Olof Johansson <olof at lixom.net>
> ---
>  arch/arm64/include/asm/processor.h |    4 ++--
>  arch/arm64/kernel/asm-offsets.c    |    2 --
>  arch/arm64/kernel/entry.S          |   34 ++++++++++++++++------------------
>  arch/arm64/kernel/process.c        |    3 ++-
>  4 files changed, 20 insertions(+), 23 deletions(-)

So why not pass in 'thread_struct' as the patch below does - it looks much simpler 
to me. This way the assembly doesn't have to be changed at all.

Thanks,

	Ingo

=====================================>

* Guenter <linux at roeck-us.net> wrote:

> On Sat, Jul 18, 2015 at 04:27:17PM -0700, Guenter wrote:
> > Hi,
> > 
> > Commit 0c8c0f03e3a2 ("x86/fpu, sched: Dynamically allocate 'struct fpu'")
> > causes s390 builds in mainline to fail as follows.
> > 
> > arch/s390/kernel/traps.c: Assembler messages:
> > arch/s390/kernel/traps.c:262: Error: operand out of range
> > 	(0x00000000000023e8 is not between 0x0000000000000000 and 0x0000000000000fff)
> > arch/s390/kernel/traps.c:300: Error: operand out of range
> > 	(0x00000000000023e8 is not between 0x0000000000000000 and 0x0000000000000fff)
> > 
> 
> Also:
> 
> arm64:allmodconfig:
> 
> arch/arm64/kernel/entry.S: Assembler messages:
> arch/arm64/kernel/entry.S:588: Error: immediate out of range
> arch/arm64/kernel/entry.S:597: Error: immediate out of range
> make[1]: *** [arch/arm64/kernel/entry.o] Error 1
> 
> I didn't bisect that one, but it looks like the cause is the same.

Hm, it looks like the new, increased offset of 'thread_struct' within 
'task_struct' goes over a limit that these instructions are able to support on 
arm64:

 arch/arm64/kernel/asm-offsets.c:  DEFINE(THREAD_CPU_CONTEXT,    offsetof(struct task_struct, thread.cpu_context));
 arch/arm64/kernel/entry.S:      add     x8, x0, #THREAD_CPU_CONTEXT
 arch/arm64/kernel/entry.S:      add     x8, x1, #THREAD_CPU_CONTEXT

If there's no instruction that can support such offset sizes then I suspect the 
straightforward fix would be to pass in thread_struct instead - like the patch 
below. That's a tiny bit cleaner for type encapsulation anyway.

Warning: it's not even build tested, but in case it works:

  Signed-off-by: Ingo Molnar <mingo at kernel.org>

Thanks,

	Ingo

================

 arch/arm64/include/asm/processor.h | 4 ++--
 arch/arm64/kernel/asm-offsets.c    | 2 +-
 arch/arm64/kernel/process.c        | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index e4c893e54f01..890f84bb3b8c 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -152,8 +152,8 @@ static inline void cpu_relax(void)
 #define cpu_relax_lowlatency()                cpu_relax()
 
 /* Thread switching */
-extern struct task_struct *cpu_switch_to(struct task_struct *prev,
-					 struct task_struct *next);
+extern struct task_struct *cpu_switch_to(struct thread_struct *prev,
+					 struct thread_struct *next);
 
 #define task_pt_regs(p) \
 	((struct pt_regs *)(THREAD_START_SP + task_stack_page(p)) - 1)
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index c99701a34d7b..3785373c2369 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -39,7 +39,7 @@ int main(void)
   DEFINE(TI_TASK,		offsetof(struct thread_info, task));
   DEFINE(TI_CPU,		offsetof(struct thread_info, cpu));
   BLANK();
-  DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct task_struct, thread.cpu_context));
+  DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct thread_struct, cpu_context));
   BLANK();
   DEFINE(S_X0,			offsetof(struct pt_regs, regs[0]));
   DEFINE(S_X1,			offsetof(struct pt_regs, regs[1]));
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 223b093c9440..436e95bda1b2 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -325,7 +325,7 @@ struct task_struct *__switch_to(struct task_struct *prev,
 	dsb(ish);
 
 	/* the actual thread switch */
-	last = cpu_switch_to(prev, next);
+	last = cpu_switch_to(&prev.thread, &next.thread);
 
 	return last;
 }