问题
Some built-in perf events are mapped to offcore events. For example, LLC-loads and LLC-load-misses are mapped to OFFCORE_RESPONSE. events. This can be easily determined as discussed in here. However, these offcore events require writing certain values to certain MSR registers to actually specify a particular event. perf seems to be using an array called something like snb_hw_cache_extra_regs to specify what values to write to which MSR registers. I would like to know how this array is used. Basically I want to know how to determine the specific offcore event that a perf event is mapped to by looking at the source code.
回答1:
The array of the offcore PM MSRs is stored here for SnB, other architectures are close.
The macro INTEL_UEVENT_EXTRA_REG is here and it just sets the msr to the second parameter, the config_mask to 0x000000FFULL and the valid_mask to the third parameter (the first being the event to program the the associated ordinary PMR with).
The values of the snb_hw_cache_extra_regs is just what it's going to be written in the selected PM MSR1.
This array is looked up here and used
here to select the first PM MSR matching, where the config arg comes from the array hw_cache_event_ids.
So it's basically a loop testing a couple of bitmasks until a compatible PM MSR is found, I think you can just skip that and look at the values stored in the snb_hw_cache_extra_regs array.
1
Since the macro defined to make the values in snb_hw_cache_extra_regs are never reused in source and the values maps directly to the MSR layout documented in the Intel manuals, I didn't bother verifying this statement.
来源：https://stackoverflow.com/questions/54223074/how-does-perf-use-the-offcore-events