Reply To: Performance counters

I had the same issues with post editing. Sometimes I will edit the post minute after posting, and after saving it will then disappear.

Ability to make some operations privileged (not just by OS, but by any process) seams nice on the surface (i.e. i.e. in some virtualization scenario it would be nice to disable wide vector FPUs ops – not just on side of specializer, but actually to trap in CPU when used; or to disable explicit instruction/data cache flushing). I understand the concern of hiding performance counters. But even with performance counters being privileged and not accessible to normal programs, there are ways to get accurate timings from user space and execute side channel attack successfully. Example: Two threads, one spinning and updating a counter in memory or L3 cache. Calibrated using normal timers or real clock (i.e. with just milisecond or second accuracy even!). Another thread reading this data back on start and end. Aka you just recreated very precise (accurate to even few cycles probably) timer. Even more accurate if you make your threads have thread affinity and be close by on the chip and sharing L2 or L3 in specific way, and not migrating often. This is very easy to accomplish.

Trying to hide timers is just a workaround, not a real solution. Making access to very accurate hardware counters will just makes more trouble in normal use.

Also, notice that I do not ask for “absolute” timer. I am specifically asking for a timer that is only progressing during execution of specific thread (turf), or a facilities to do so. This way it is immune to what other threads are doing to big extent, even if the turf was context switched to something else, and then back. It will still see L1/L2 cache latencies if it was touched by other thread during its own execution of course. I see absolutely no way to prevent that in general. Meltdown is easy to fix by other means. Spectre can be addressed by compiler and hardware too, and Mill does do few tricks to make it work well without any impact on performance.