Hi Dan,
It is likely that the instruction that is causing the fault is not being decoded by hardware or software. ARM has done a reasonable job in providing hardware support for decoding faulting instructions, but only a subset of the ISA is supported.
The instruction causing the fault might be a load with increment (*pArray++). A trivial fix might be to rewrite your code as:
#define idx(i) (i%16)
for(i = 0; i < buf->capacity() && buf->capacity() > 16; i++){
mem[idx(i)] = *pArray[i];
}
Failing that, I recommend using objdump to identify the faulting instruction and follow the code path in ../projects/seL4_projects_libs/libsel4arm-vmm/src/fault.c: get_rt: 357 to see how that particular instruction is being decoded and handled.
- Alex Kroh
From: Devel on behalf of Dan DaCosta
Sent: Saturday, September 8, 2018 4:19 AM
To: devel@sel4.systems
Cc: Darren Cofer
Subject: [seL4] Strange behavior when writing to memory mapped to a custom device in the VMM.
To Whom It May Concern:
I am observing seL4+VM+ARM behavior on the TK1-SOM that I find very
puzzling. Consider the following code:
#define idx(i) (i%16)
for(i = 0; i < buf->capacity() && buf->capacity() > 16; i++, pArray++){
mem[idx(i)] = *pArray;
}
where 'pArray' is the array representation of 'buf' and 'mem' is a
memory mapped page set that corresponds to a page set of memory that
is managed by a fault handler in the VMM (something I have installed
myself using a 'DEV_CUSTOM' device). This code runs as I would expect.
However, the follow code results in an assertion failure in the VMM:
#define idx(i) (i%16)
for(i = 0; i < 16; i++, pArray++){
mem[idx(i)] = *pArray;
}
The error report follows:
32 bit ARM insts not decoded
--------
Pagefault from [Linux]: write fault @ PC: 0xbd7499a8 IPA: 0xe0001000, FSR: 0x2000046
Context:
r0: 0xb6d2a050
r1: 0x0
r2: 0xb6ac76b8
r3: 0x4c
r4: 0xb6d2a050
r5: 0xb6d2a8ec
r6: 0x1
r7: 0xb6d2a050
r8: 0x7fb73000
r9: 0xad13cfff
r10: 0xabe31310
r11: 0x19
r12: 0xb6d285d8
pc: 0x7f68f9a8
r14: 0x7f68f99c
sp: 0xaf1c3c20
cpsr: 0x80000010
--------
Assertion failed: rt >= 0 (../projects/seL4_projects_libs/libsel4arm-vmm/src/fault.c: get_rt: 357)
Some added information that may be helpful; if the strange modulo
operation I am performing on the index is removed I receive a similar
crash for both code snippets.
Any thoughts on where I might look to get to the bottom of this problem
would be helpful.
Thanks!
Dan DaCosta
