Both C3 and C6 (and their package state limits) are broken. Both should be disabled and can lead to a lockup. You can read about it in the Intel Whitepapers.
https://www.intel.com/content/dam/w...cification-updates/xeon-e5-v3-spec-update.pdf
See HSE20, HSE 25 and HSE49 (without C3 no re-entering of C1E).
All three I can observe if I have any C state activated except C1E. As you can see in the revision history, there never came a fix (like with almost all their CPUs).
Let me add my experience.
Having enabled C6 state can and does result in BSOD at idle and/or light loading, regardless of manner of operation.
Typically, I do not care to enable C3; however...
There is a most definitely an OR reference for C3 and C6 states hard-coded into the Power Control Unit (PCU) logic that if neither is enabled limits how cores with turbo boost as a function of loading. If you enabled and disable C3 and run a number of workloads while monitoring with a per-core frequency monitor like HWiNFO64 you will see the difference and it is recognizable and quantifiable.
Furthermore, if you then set the Package C state limit to C2 or lower (I recommend C0/C1 as
most of the savings is at C1) I find there is no problem having C3 enabled and I get the best of both world... the higher turbo boosts (higher performance) without the deleterious effects of C3/C6 enabled.
Finally, for this to be entirely successful, I should mention that I must set the Windows Power Plan to High Performance. Balanced Performance perhaps allows C3 at the core level (which cannot be stopped with the in-BIOS
Package limit) and BSOD can result. Ergo, keep Power Plan on High Performance and tweek that profile as necessary. Yes, slightly higher power usage but then it's performance we're after, not efficiency.
EDP, electrical design point (CPU hardlimit). There are many suggestions for a workaround (mostly by me in previous posts), but no one managed to succeed one so far. I guess mostly it is just too time consuming and unpredictable in the harm to the hardware. An EDP exist for a reason, you know ?
Nearest I can tell EDP is a hardwired (fused) limit set at time of binning must like how multipliers are set. There's most likely a set of registered banks that are addressed at time of product identification that cannot be changed once set. PCU logic references these registers and imposes final limits. My attempts to defeat have been utterly defeated.
EDIT: Add reference to Windows power plan settings