Simultaneous Write/Read to on chip dual port RAM

You can safely detect 'changed' on a word even without OLD_DATA. In the simplest case if the word changes from 0 to 1 (and back) I don't believe you'll ever read an illegal value.

So you could, for example, detect the last word of the buffer being changed by writing a reserved value to it, and waiting for it to change.

But why not put the control bit somewhere outside the shared memory - like a PIO bit in the avalon slave with the control registers for your logic that is writing the buffers?

Then your control loop polls the pio slave to determine when the buffer is available.