The ARM architecture requires all accesses to device memory to be naturally aligned[1][2]. Normal memory does not have this strict requirement, and in fact many systems do ignore unaligned accesses (by the means of clearing the A bit in SCTLR and accessing normal memory). So the default behaviour of GCC assumes that unaligned accesses are fine, at least if happening on the stack. Now kvm-unit-tests runs some C code with the MMU off, which degrades the whole system memory to device memory. Now every unaligned access will fault, regardless of the A bit. In fact there is at least one place in lib/printf.c where GCC merges two consecutive char* accesses into one "strh" instruction, writing to a potentially unaligned address. This can be reproduced by configuring kvm-unit-tests for kvmtool, but running it on QEMU, which triggers an early printf that exercises this particular code path. Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this problem. Also add the respective -mno-unaligned-access flag for arm. Thanks to Alexandru for helping debugging this. Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx> [1] ARMv8 ARM DDI 0487E.a, B2.5.2 [2] ARMv7 ARM DDI 0406C.d, A3.2.1 --- arm/Makefile.arm | 1 + arm/Makefile.arm64 | 1 + 2 files changed, 2 insertions(+) diff --git a/arm/Makefile.arm b/arm/Makefile.arm index a625267..43b4be1 100644 --- a/arm/Makefile.arm +++ b/arm/Makefile.arm @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y CFLAGS += $(machine) CFLAGS += -mcpu=$(PROCESSOR) +CFLAGS += -mno-unaligned-access arch_LDFLAGS = -Ttext=40010000 diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64 index 02c24e8..35de5ea 100644 --- a/arm/Makefile.arm64 +++ b/arm/Makefile.arm64 @@ -7,6 +7,7 @@ bits = 64 ldarch = elf64-littleaarch64 arch_LDFLAGS = -pie -n +CFLAGS += -mstrict-align define arch_elf_check = $(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"), -- 2.17.1 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
