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
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 <devel-bounces@sel4.systems> on behalf of Dan DaCosta <chaosape@gmail.com> 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
participants (2)
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Alexander.Kroh＠data61.csiro.au -
Dan DaCosta