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. Thomas _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
