add msgq readme

messaging/msgq.md

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+# MSGQ: A lock free single producer multi consumer message queue
+
+[![Build Status](https://dev.azure.com/commaai/default/_apis/build/status/commaai.msgq?branchName=master)](https://dev.azure.com/commaai/default/_build/latest?definitionId=21&branchName=master)
+
+## What is MSGQ?
+MSGQ is a system to pass messages from a single producer to multiple consumers. All the consumers need to be able to receive all the messages. It is designed to be a high performance replacement for ZMQ-like SUB/PUB patterns. It uses a ring buffer in shared memory to efficiently read and write data. Each read requires a copy. Writing can be done without a copy, as long as the size of the data is known in advance.
+
+## Storage
+The storage for the queue consists of an area of metadata, and the actual buffer. The metadata contains:
+
+1. A counter to the number of readers that are active
+2. A pointer to the head of the queue for writing. From now on referred to as *write pointer*
+3. A cycle counter for the writer. This counter is incremented when the writer wraps around
+4. N pointers, pointing to the current read position for all the readers. From now on referred to as *read pointer*
+5. N counters,  counting the number of cycles for all the readers
+6. N booleans, indicating validity for all the readers. From now on referred to as *validity flag*
+
+The counter and the pointer are both 32 bit values, packed into 64 bit so they can be read and written atomically.
+
+The data buffer is a ring buffer. All messages are prefixed by an 8 byte size field, followed by the data. A size of -1 indicates a wrap-around, and means the next message is stored at the beginning of the buffer.
+
+
+## Writing
+Writing involves the following steps:
+
+1. Check if the area that is to be written overlaps with any of the read pointers, mark those readers as invalid by clearing the validity flag.
+2. Write the message
+3. Increase the write pointer by the size of the message
+
+In case there is not enough space at the end of the buffer, a special empty message with a prefix of -1 is written. The cycle counter is incremented by one. In this case step 1 will check there are no read pointers pointing to the remainder of the buffer. Then another write cycle will start with the actual message.
+
+There always needs to be 8 bytes of empty space at the end of the buffer. By doing this there is always space to write the -1.
+
+## Reset reader
+When the reader is lagging too much behind the read pointer becomes invalid and no longer points to the beginning of a valid message. To reset a reader to the current write pointer, the following steps are performed:
+
+1. Set valid flag
+2. Set read cycle counter to that of the writer
+3. Set read pointer to write pointer
+
+## Reading
+Reading involves the following steps:
+
+1. Read the size field at the current read pointer
+2. Read the validity flag
+3. Copy the data out of the buffer
+4. Increase the read pointer by the size of the message
+5. Check the validity flag again
+
+Before starting the copy, the valid flag is checked. This is to prevent a race condition where the size prefix was invalid, and the read could read outside of the buffer. Make sure that step 1 and 2 are not reordered by your compiler or CPU.
+
+If a writer overwrites the data while it's being copied out, the data will be invalid. Therefore the validity flag is also checked after reading it. The order of step 4 and 5 does not matter.
+
+If at steps 2 or 5 the validity flag is not set, the reader is reset. Any data that was already read is discarded. After the reader is reset, the reading starts from the beginning.
+
+If a message with size -1 is encountered, step 3 and 4 are replaced by increasing the cycle counter and setting the read pointer to the beginning of the buffer. After that another read is performed.