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

[Will Deacon <will.deacon@arm.com>]

On Mon, Jul 20, 2015 at 08:36:47AM +0100, Ingo Molnar wrote:
> * Olof Johansson <olof@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@linux.intel.com>
> > Signed-off-by: Olof Johansson <olof@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.

Unfortunately, neither of these approaches really work:

  - We need to return last from __switch_to, which means not corrupting
    x0 in cpu_switch_to and then having an ugly container_of to get back
    at the task_struct

  - ret_from_fork needs to pass the task_struct of prev to schedule_tail,
    so we have the same issue there

Patch below fixes things, but it's a shame we have to use an extra register
like this.

Will

--->8

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f860bfda454a..e16351819fed 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -585,7 +585,8 @@ ENDPROC(el0_irq)
  *
  */
 ENTRY(cpu_switch_to)
-	add	x8, x0, #THREAD_CPU_CONTEXT
+	mov	x10, #THREAD_CPU_CONTEXT
+	add	x8, x0, x10
 	mov	x9, sp
 	stp	x19, x20, [x8], #16		// store callee-saved registers
 	stp	x21, x22, [x8], #16
@@ -594,7 +595,7 @@ ENTRY(cpu_switch_to)
 	stp	x27, x28, [x8], #16
 	stp	x29, x9, [x8], #16
 	str	lr, [x8]
-	add	x8, x1, #THREAD_CPU_CONTEXT
+	add	x8, x1, x10
 	ldp	x19, x20, [x8], #16		// restore callee-saved registers
 	ldp	x21, x22, [x8], #16
 	ldp	x23, x24, [x8], #16