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Haswell bug leads Intel to disable TSX

Haswell bug leads Intel to disable TSX

A bug in the Haswell microarchitecture has led Intel to disable transactional memory (TSX) support in all chip editions, with no word yet on when a fixed stepping will be available.

Intel has admitted that a design flaw in its latest Haswell microarchitecture has led to the disabling of transactional memory extensions, known as Intel TSX, in all versions of the chips.

Intel TSX was touted as a major feature of the new Haswell microarchitecture when it was unveiled in February 2012. The system works by allowing programmers to specify memory locations which should be synchronised for transactional use; then, like a database, operations can be performed in isolation without the fear that they will become unsynchronised. For multi-threaded applications written with TSX in mind, the technology promises a considerable performance boost as unnecessary synchronisation steps normally required in threaded programming can be removed in favour of TSX.

Sadly, there's a problem: a bug has crept into the Haswell microarchitecture that means TSX doesn't work properly, and Intel has been forced to disable TSX altogether via a microcode update. According to The Tech Report, this update applies to all Haswell-based processors and will completely lock off TSX support.

Intel has confirmed the existence of the flaw, which has been officially recorded as erratum for all existing steppings of the Haswell microarchitecture. A fix for the issue, which was discovered by a developer trying to use TSX outside Intel, is planned for future steppings - but, for now, there are no Haswell chips on the market or planned in the immediate future that will include a working implementation of TSX.

Intel has not indicated how it plans to address complaints from buyers who specifically opted for Haswell chips in order to use TSX in their applications.

10 Comments

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Corky42 13th August 2014, 13:25 Quote
With Haswell over two years old I'm surprised a bug has been discovered after all this time in the market, with Broadwell just around the corner I'm also surprised they are planning of fixing it versus just leaving TSX disabled. It does raise the question that if the bug has only just been discovered has it also crept into Broadwell ?
Pookie 13th August 2014, 13:28 Quote
So how do Intel disable all the TSX Haswell chips out there already? Is it just disabled by driver updates?
Gareth Halfacree 13th August 2014, 13:30 Quote
Quote:
Originally Posted by Pookie
So how do Intel disable all the TSX Haswell chips out there already? Is it just disabled by driver updates?
Microcode updates. They'll be built into motherboards from now on, shipped as BIOS updates to existing motherboards, and through operating system updates like Windows Update.
Corky42 13th August 2014, 13:31 Quote
Normally the microcode is updated when the BIOS is flashed, AFAIK.

Darn Mr G beat me to it ;) And with a more correct answer to boot.
Nexxo 13th August 2014, 13:59 Quote
Well, that's embarrassing... But with the insane complexity of today's CPUs not entirely surprising.
Pookie 13th August 2014, 14:08 Quote
Quote:
Originally Posted by Gareth Halfacree
Microcode updates. They'll be built into motherboards from now on, shipped as BIOS updates to existing motherboards, and through operating system updates like Windows Update.

For a minute I imagined Intel having a secret backdoor to all the processors out there.
iggy 13th August 2014, 15:20 Quote
Product not working as intended? sounds like cause for refunds all round then.
Spreadie 13th August 2014, 22:30 Quote
So, now this "major feature" is no more, will they be dropping the price of Haswell?

Thought not.
faugusztin 13th August 2014, 23:21 Quote
Quote:
Originally Posted by Corky42
With Haswell over two years old I'm surprised a bug has been discovered after all this time in the market

Feature not used means bugs in it are not found. The fact that it was found after nearly 2 years is simply because no one really used them so far.
r3loaded 14th August 2014, 09:44 Quote
While it's too late for Intel to fix TSX support in Haswell, Haswell-E and Haswell-EP, it'll be a consolation to know that TSX will be fixed for Haswell-EX. Though to be fair, a Haswell-EX user would be the type most likely to benefit from TSX.

Also, this is why E/EP arrives a year after the desktop/mobile chips and EX takes another year. Server customers need all the extra testing and validation that goes into them to catch really awkward bugs like these. Logic design is hard (speaking from personal experience!)
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