On 11/09/2019 10.16, Andre Przywara wrote: > On Tue, 10 Sep 2019 20:15:19 +0200 > Thomas Huth <thuth@xxxxxxxxxx> wrote: > > Hi, > >> On 05/09/2019 19.15, Andre Przywara wrote: >>> 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 >> >> Instead of adding it to both, Makefile.arm and Makefile.arm64, you could >> also simply add it to Makefile.common instead. > > But the arguments are not the same (admittedly against intuition)? > I thought about defining arch_CFLAGS in both files, then adding that to Makefile.common, but didn't see the advantage over this straightforward approach here. D'oh, never mind, I didn't read the patch properly. I somehow thought that the arguments are the same. It's quite weird that the compiler developers chose different names here... Thomas
