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(Copy of https://stackoverflow.com/questions/38868226/what-is-the-status-of-the-tsx-related-skylake-errata-skl-105 which didn't get much attention).

As is well known, Intel had to disable TSX in the Haswell-series of processors via a microcode updates. This was due to a bug in the TSX implementation that could give erroneous results if these instructions were used.

What seems to be less well known is that there is apparently also an errata affecting TSX on the newer architecture, Skylake. Specifically the errata "SKL-105" mentioned here:

http://www.intel.com/content/www/us/en/processors/core/desktop-6th-gen-core-family-spec-update.html

It specifically states that using TSX can lead to unpredictable system behavior. However, it also notes that it is possible for the BIOS to carry a fix. However, the question is then what this fix entails. Does it disable TSX altogether like the Haswell microcode "fix"? Googling "SKL105" gives no results so it seems the community is generally unaware of it?

Some users have noticed the TSX feature getting "steathily" disabled (but seemingly being unaware of the errata above):

https://www.reddit.com/r/hardware/comments/44k218/intel_disables_tsx_transactional_memory_again_in/

It is strange if only certain variants of the CPUs are affected, since one would presume they would all share the same microarchitecture and hence be equally affected by this bug.

By the way another way such a microcode "fix" could operate and which could be even more stealthy: I suppose it would be possible to make a microcode update that would still expose the presence of TSX (making it seem the feature was still enabled) but would override the implementation of the new TSX instructions with "dummy implementations" that actually would never elide the locks and essentially just execute the code the old-fashioned way, thereby avoiding the bug but also foregoing the performance improvement TSX could offer. The only way to determine if this happened would be through performance measurements.

Anyone has more info on the status of TSX in Skylake? In any case it is strange that not more info is released and one has to guess what is affected and what is not. And indeed if the feature is safe to use.

I have a 6700K and the feature is still there. But this also depends on whether the BIOS manufacturer has taken in the microcode updates and also I haven't actually measured the performance so I can't exclude it could still have been disabled cf. the previous paragraph.

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closed as off-topic by D.W. Aug 11 '16 at 21:38

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  • $\begingroup$ Also posted on SO. Please do not post the same question on multiple sites. Each community should have an honest shot at answering without anybody's time being wasted. $\endgroup$ – D.W. Aug 11 '16 at 21:39
  • $\begingroup$ Also, questions about specific products or software platforms are off-topic here. $\endgroup$ – D.W. Aug 11 '16 at 21:39