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target/riscv: warn about truncating register values #1268

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target/riscv: warn about truncating register values #1268

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31 changes: 24 additions & 7 deletions src/target/riscv/riscv-013.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1370,14 +1370,31 @@ static int register_read_progbuf(struct target *target, uint64_t *value,
{
assert(target->state == TARGET_HALTED);

if (number >= GDB_REGNO_FPR0 && number <= GDB_REGNO_FPR31)
return fpr_read_progbuf(target, value, number);
else if (number >= GDB_REGNO_CSR0 && number <= GDB_REGNO_CSR4095)
return csr_read_progbuf(target, value, number);
int res;
if (number >= GDB_REGNO_FPR0 && number <= GDB_REGNO_FPR31) {
res = fpr_read_progbuf(target, value, number);
} else if (number >= GDB_REGNO_CSR0 && number <= GDB_REGNO_CSR4095) {
res = csr_read_progbuf(target, value, number);
} else {
LOG_TARGET_ERROR(target, "Unexpected read of %s via program buffer.",
riscv_reg_gdb_regno_name(target, number));
return ERROR_FAIL;
}
if (res != ERROR_OK)
return res;

LOG_TARGET_ERROR(target, "Unexpected read of %s via program buffer.",
riscv_reg_gdb_regno_name(target, number));
return ERROR_FAIL;
unsigned int size_bits = register_size(target, number);
unsigned int value_bits = sizeof(*value) * CHAR_BIT;
assert(size_bits <= value_bits);
if (size_bits == value_bits || *value >> size_bits == 0)
return ERROR_OK;

LOG_TARGET_WARNING(target, "Value read for register %s of %" PRIx64
" does not fit into the size of the register (%u bits)."
" This is a HW bug. Truncating the value to fit into the register.",
riscv_reg_gdb_regno_name(target, number), *value, size_bits);
Comment on lines +1392 to +1395
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This is a copy of #1233 (comment) by @JanMatCodasip:

If this HW problem occurs, it is a serious thing, and we better not trust the returned value at all.

Instead of truncating the value, I would recommend to change the message level to ERROR and end the operation with an error code.

Few additional details:

  • Added prefix 0x in front of the hex number
  • I'd suggest minor change of word order for readability
  • does not fit into the size --> exceeds the size (shorter and more formal)
Suggested change
LOG_TARGET_WARNING(target, "Value read for register %s of %" PRIx64
" does not fit into the size of the register (%u bits)."
" This is a HW bug. Truncating the value to fit into the register.",
riscv_reg_gdb_regno_name(target, number), *value, size_bits);
LOG_TARGET_ERROR(target, "Value 0x%" PRIx64 " read from register %s"
" exceeds the size of the register (%u bits). This is a HW bug. "
*value, riscv_reg_gdb_regno_name(target, number), size_bits);
return ERROR_FAIL;

Otherwise this change looks good.

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I agree that this should be an error. While we do accommodate some targets which aren't fully RISC-V debug compliant (notably GD32VF103), this is serious and we can't expect anything.

*value &= GENMASK_ULL(size_bits - 1U, 0U);
return ERROR_OK;
}

/**
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