Building a Linux powernv kernel with clang

These instructions are current as of v4.19-rc2 running Ubuntu 18.10 with clang-7 (7.0.0-+rc2-1~exp3). This will work on a ppc64le box or cross compiled from another machine.

export ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu-
make powernv_defconfig

Disable PPC_DISABLE_WERROR due to -Wduplicate-decl-specifier warnings in arch/powerpc/include/asm/uaccess.h (see https://github.com/linuxppc/linux/issues/185)
Disable FTRACE due to this enabling the -mno-sched-epilog option which clang does not know about.
Disable MD_RAID456 and BTRFS_FS as lib/raid6/altivec1.c causes clang to crash
Apply this patch https://patchwork.ozlabs.org/patch/966825/:

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 11a1acba164a..a70639482053 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -238,7 +238,7 @@ cpu-as-$(CONFIG_4xx)                += -Wa,-m405
 cpu-as-$(CONFIG_ALTIVEC)       += $(call as-option,-Wa$(comma)-maltivec)
 cpu-as-$(CONFIG_E200)          += -Wa,-me200
 cpu-as-$(CONFIG_E500)          += -Wa,-me500
-cpu-as-$(CONFIG_PPC_BOOK3S_64) += -Wa,-mpower4
+cpu-as-$(CONFIG_PPC_BOOK3S_64) += $(call as-option,-Wa$(comma)-mpower8,-Wa$(comma)-mpower4)
 cpu-as-$(CONFIG_PPC_E500MC)    += $(call as-option,-Wa$(comma)-me500mc)

 KBUILD_AFLAGS += $(cpu-as-y)

make CC=clang-7 CLANG_TRIPLE=powerpc64le-linux-gnu

And then the link step will fail with this:

ld: kernel/irq/manage.o: in function `local_bh_enable':
manage.c:(.text+0x3ccc): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+3cb8
ld: kernel/rcu/srcutree.o: in function `local_bh_enable':
srcutree.c:(.text+0x258c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+2578
ld: kernel/time/hrtimer.o: in function `local_bh_enable':
hrtimer.c:(.text+0x242c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+2418
ld: kernel/bpf/cpumap.o: in function `local_bh_enable':
cpumap.c:(.text+0x134c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+1338
ld: drivers/net/ethernet/broadcom/tg3.o: in function `local_bh_enable':
tg3.c:(.text+0x87dc): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+87c8
ld: drivers/net/ethernet/intel/e1000/e1000_main.o: in function `local_bh_enable':
e1000_main.c:(.text+0xc13c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+c128
ld: net/core/sock.o: in function `local_bh_enable':
sock.c:(.text+0x5a6c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+5a58
ld: net/core/gen_estimator.o: in function `local_bh_enable':
gen_estimator.c:(.text+0x2ec): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+2d8
ld: net/core/dev.o: in function `local_bh_enable':
dev.c:(.text+0x8a6c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+8a58
ld: net/core/neighbour.o: in function `local_bh_enable':
neighbour.c:(.text+0x1f2c): relocation truncated to fit: R_PPC64_ADDR16_HA against `.text'+1f18
ld: net/sched/sch_generic.o: in function `local_bh_enable':
sch_generic.c:(.text+0x392c): additional relocation overflows omitted from the output

Nick suggested this was due to the use of _THIS_IP_, which takes the address of a local label in order to create a pointer to the current program counter (I think). GCC generates a TOC entry for this, but clang is trying to fit it in

A bug was reported and a patch posted.

0000000000002400 <local_bh_enable>:
    2400:       00 00 4c 3c     addis   r2,r12,0
    2404:       00 00 42 38     addi    r2,r2,0
    2408:       a6 02 08 7c     mflr    r0
    240c:       10 00 01 f8     std     r0,16(r1)
    2410:       e1 ff 21 f8     stdu    r1,-32(r1)
    2414:       00 00 60 38     li      r3,0
    2418:       00 02 80 38     li      r4,512
    241c:       00 00 63 3c     addis   r3,r3,0
    2420:       01 00 00 48     bl      2420 <local_bh_enable+0x20>
    2424:       00 00 00 60     nop
    2428:       20 00 21 38     addi    r1,r1,32
    242c:       10 00 01 e8     ld      r0,16(r1)
    2430:       a6 03 08 7c     mtlr    r0
    2434:       20 00 80 4e     blr
        ...
    2444:       00 00 00 60     nop
    2448:       00 00 00 60     nop
    244c:       00 00 42 60     ori     r2,r2,0

(disassembly of kernel/softirq.o built with clang)

Written on September 6, 2018