Hi,
On Thu Mar 21, 2013 at 13:55:29 +0400, Sergey Grekhov wrote:
I have a question regarding method Kobject_iface::commit_result() in the code of Fiasco.OC. One can easily find in the code something similar to following:
return commit_result(-L4_err::EPerm);
And this call is very confusing: it looks like returning message tag with error, but in fact the error bit (0x8000) is not set. Moreover, some values from L4_err is intersection with enumeration L4_msg_tag::Protocol because of:
- sign '-' used when commiting result;
- place where both values are saved - bits 31...16 of the message
tag. And, finally, there are lots of returns similar to mentioned one (return commit_result(-L4_err::EPerm)) and there are no _any_ code which checks presence of these errors in the tag.
I am very confused about these strange values saved in L4_msg_tag and it appears to me that in these cases should be used method Kobject_iface::commit_error(). Can someone explain the logic behind returning values defined in class L4_err?
The error bit 0x8000 indicates an IPC error, i.e. an error related to communication. Just using commit_result() returns a return code to the caller, also meaning that the IPC itself was ok. And indeed, the label field in the msg_tag is used for both transmitting a protocol number to the callee and the return code back to the caller, that's why they are also negative to indicate errors. The msg_tag error bit is checked in userland, see l4_ipc_error() and l4_error() functions in l4sys.
Adam