target/riscv: warn about truncating register values

[en-sc]

When reading a register via Program Buffer the actual value is read from a GPR. The GPR size (XLEN) can be greater than the length of the target register (e.g. dcsr is always 32-bit wide). Due to some HW issue GPR read may return a value that needs to be truncated to fit into the target register. Warn the user about it.

Moreover, the value in cache was and is truncated, but the riscv_reg_t that is filled by riscv_reg_get() didn't use to be. This may lead to an assertion failure in abstract_data_write_fill_batch(): Link:

riscv-openocd/src/target/riscv/riscv-013.c

Line 757 in fa7e235

assert(value <= UINT32_MAX || size_in_words > 1);

Change-Id: I4d8a5ba2451fc5a60f51b9143b6b140dfe3b73b8

[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.