Re: x86/non-x86: percpu, node ids, apic ids x86.git fixup

Ingo Molnar
2008-01-30T20:59:50Z

* Luck, Tony <tony.luck@intel.com> wrote:

> > could you send the .config you are using?
>
> Ok. Attached.

thanks a ton - this produced a link error here too.

after half an hour of head scratching, the updated patch below solves
the build problem.

The problem i believe is this code in arch/ia64/kernel/mca_asm.S:

#define GET_IA64_MCA_DATA(reg) \
GET_THIS_PADDR(reg, ia64_mca_data) \
;; \
ld8 reg=[reg]

this i believe builds an implicit dependency between the mca_asm.o
position within the image and the ia64_mca_data percpu variable it
accesses - it relies on the immediate 22 addressing mode that has 4MB of
scope. Per chance, the .config you sent creates a 14MB image, and the
percpu variables moved too far away for the linker to be able to fulfill
this constraint.

The workaround is to define PER_CPU_ATTRIBUTES to link percpu variables
back into the .percpu section on UP too - which ia64 links specially
into its vmlinux.lds. But ultimately i think the better solution would
be to remove this dependency between arch/ia64/kernel/mca_asm.S and the
position of the percpu data.

Is my analysis correct? Do you like my fix and does the patch build and
boot on your system? Thanks,

Ingo

--------------->
Subject: ia64: on UP percpu variables are not small memory model
From: Ingo Molnar <mingo@elte.hu>

Tony says:

| The CONFIG_SMP=n path in ia64 makes quite radical changes ... rather
| than putting all the per-cpu stuff into the top 64K of address space
| and providing a per-cpu TLB mapping for that range to a different
| physical address ... it just makes all the per-cpu stuff link as ordinary
| variables in .data.

the new generic percpu code got confused about this as PER_CPU_ATTRIBUTES
was defined even on UP, so it picked up that small memory model - which
was not possible to get linked. The right fix is to only define that
on SMP. This resolved the build failures in my cross-compiling environment.

also link these variables into the .percpu section - some assembly code
has offset dependencies.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/asm-ia64/percpu.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-x86.q/include/asm-ia64/percpu.h
===================================================================
--- linux-x86.q.orig/include/asm-ia64/percpu.h
+++ linux-x86.q/include/asm-ia64/percpu.h
@@ -15,18 +15,20 @@

#include <linux/threads.h>

+#ifdef CONFIG_SMP
+
#ifdef HAVE_MODEL_SMALL_ATTRIBUTE
# define PER_CPU_ATTRIBUTES __attribute__((__model__ (__small__)))
#endif

-#ifdef CONFIG_SMP
-
#define __my_cpu_offset __ia64_per_cpu_var(local_per_cpu_offset)

extern void *per_cpu_init(void);

#else /* ! SMP */

+#define PER_CPU_ATTRIBUTES __attribute__((__section__(".data.percpu")))
+
#define per_cpu_init() (__phys_per_cpu_start)

#endif /* SMP */