End of unofficial use of the AVX-512 on Alder Lake. Intel will blow it up with an update

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If you’re pleased that the 512-bit SIMD AVX-512 instructions at Alder Lake can unofficially turn on, even though the processor doesn’t support them, bad news. Intel will cancel this option.

When they came out the first Alder Lake processors (125W models for enthusiasts), it quickly became clear that although according to Intel they should have AVX-512 shut down permanently for Golden Cove cores, in fact, was turned off quite reversibly in the firmware. The BIOS of many boards allowed it to turn on when the user deactivated E-Core. To the delight of those who wanted 512-bit instructions. Unfortunately, it looks like the celebrations were premature and Intel will cancel this bonus again.

As we wrote in November, Intel at Alder Lake processors so far he did not turn off the AVX-512 so that it would completely disable it in the factory by some physical intervention (although it is possible that some future revision will introduce this). The shutdown was part of a low-level configuration set by the processor’s BIOS. Intel has tried to hide the existence of this mechanism in the code it supplies to the board manufacturers, but failed, and the board manufacturers were able to turn the AVX-512 back on before the processors were released. You will find this option in the Z690 board BIOS if you deactivate all E-Cores on the processor. The AVX-512 then works normally in applications as well and Rocket Lake. However, there is no guarantee that it will work properly.

Upcoming microcode updates will prevent the use of the AVX-512

Unfortunately, Intel does not seem to like this tolerance and will not tolerate it. Igor’sLAB website informs that Intel is going to update the microcode of the processors, which are supposed to disable this trick again and will probably remove the AVX-512 in some other way. Microcode updates are like updating the firmware of the CPU itself, so they can change how it works. Unfortunately, when this update arrives, the semi-official AVX-512 will be removed. Unfortunately, this change will also affect previously purchased processors, not just some new ones sold after this update.

As you probably know, the microcode update has the specificity that it is not permanently loaded in the processor, it must be loaded by either the BIOS board or the operating system each time it is started or restarted. This means that if you do not intend to give up AVX-512 support, you have the option of disabling the upload of new microcodes to the operating system (in Windows you must disable and uninstall the update that takes care of this; on Linux the microcode is usually distributed from repositories package).

However, because the new microcodes will also be part of the newly released board BIOSes, you will also need to stop updating the board BIOS (UEFI) and stay on the current version. If future versions add some important features or fixes, you will need to regret them. You will also potentially lose security fixes if future BIOSes or microcode updates through the OS fix any security holes.

According to Igor’sLAB, unofficially modified BIOSes have some hope. Sometimes it should be possible to take a newer BIOS and replace the new processor microcode with the old version. This is a binary object that loads itself into the CPU and does not interact with the BIOS code itself. However, this has the disadvantage of all BIOS modifications – increased risk of errors or problems and then also where you get such a BIOS. If you do not make the mod yourself, you run the security risk because the modified firmware from an untrusted source could theoretically contain malware / backdoor.

Too bad the AVX-512 was an extra bonus

As long as it was semi-officially possible to use the AVX-512 with Alder Lake processors, this was undoubtedly an advantage. Especially with the Core i5 processors (except for the 125W unlocked variant) and the Core i3 processors, which have only large cores and no small ones, this performance could have been more or less free (perhaps only with increased consumption as a price). Now it seems that we can no longer take this hidden bonus into account with these cheaper AVX-512 models.

It is unclear why Intel has taken such a tough stance against restarting the AVX-512. The company is obviously not willing to look at it, for example, as an overclocking and as a potential attraction for those interested in buying. It is theoretically possible that the AVX-512 implementation has some known bugs and is not fully correct, so in rare circumstances it can cause problems. Then the shutdown would be similar TSX shutdown at Skylake motivated by stability – Intel does not want rare crashes or freezes in applications to damage its reputation.

However, Igor’sLAB allegedly did not receive any reports on such a justification. There’s also a less understandable explanation in the game that Intel doesn’t want low-cost AVX-512 processors to compete with its high-performance models with these extensions, such as Ice Lake-SP. However, this explanation is somewhat weakened by the fact that there are many older processors that the AVX-512 had, such as Tiger Lake or Rocket Lake. It’s also possible that Intel is fighting a bit of “purist” motives with hybrid AVX-512s – if the AVX-512 only had cheap Core i3 and i5 models and not i7 and i9, it would look inelegant. And similarly, you may not like it when you turn off small E-Cores. In the eyes of customers, this may create a tendency to take a negative view of hybrid technology and an aversion to buying more expensive Intel CPUs, which all include E-Core.

Anyway, if you use AVX-512 either because it helps you in some specific software (one such use is in the RPCS3 emulator, for example), or because you are a programmer (or a programmer, of course), then you will have to watch out for updates. Or possibly switch to another CPU.

Source: igor’sLAB