Hello!
> > The problem has been discovered by performing an operation
> >
> > *((volatile int *)reg) = 0;
> >
> > which compiles as "str xzr, [xx]", and resulted in strange values being
> > written.
>
> Interesting find. Which compiler is that?
$ aarch64-linux-gnu-gcc --version
aarch64-linux-gnu-gcc (Linaro GCC 2014.11) 4.9.3 20141031 (prerelease)
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
This is from my colleague who actually hit the bug by his driver. And i can
reproduce the issue with different compiler version
using the following small testcase:
--- cut ---
p.fedin@fedinw7x64 /cygdrive/d/Projects/Test
$ cat test.c
volatile int *addr;
int test_val(int val)
{
*addr = val;
}
int test_zero(void)
{
*addr = 0;
}
p.fedin@fedinw7x64 /cygdrive/d/Projects/Test
$ aarch64-unknown-linux-gnu-gcc -O2 -c test.c
p.fedin@fedinw7x64 /cygdrive/d/Projects/Test
$ aarch64-unknown-linux-gnu-objdump -d test.o
test.o: file format elf64-littleaarch64
Disassembly of section .text:
0000000000000000 <test_val>:
0: 2a0003e2 mov w2, w0
4: 2a0103e0 mov w0, w1
8: 90000001 adrp x1, 8 <test_val+0x8>
c: f9400021 ldr x1, [x1]
10: b9000022 str w2, [x1]
14: d65f03c0 ret
0000000000000018 <test_zero>:
18: 90000001 adrp x1, 8 <test_val+0x8>
1c: f9400021 ldr x1, [x1]
20: b900003f str wzr, [x1]
24: d65f03c0 ret
p.fedin@fedinw7x64 /cygdrive/d/Projects/Test
$ aarch64-unknown-linux-gnu-gcc --version
aarch64-unknown-linux-gnu-gcc (GCC) 4.9.0
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
--- cut ---
Isn't it legitimate to write from ZR to MMIO register?
Another potential case is in our vgic-v3-switch.S:
msr_s ICH_HCR_EL2, xzr
It's only because it is KVM code we have never discovered this problem yet.
Somebody could write such a thing in some other place,
with some other register, which would be executed by KVM, and... boo...
Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia
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