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. Cheers, Andre.
