Hacker News new | ask | show | jobs
by 0x0 3465 days ago
The last time I read about TSX it was a story about how Intel pushed a microcode update to disable TSX because it was flawed. Has this been fixed in newer CPUs? Is there a risk of TSX being flawed on CPUs in the wild (for example, if you're missing the latest microcode updates?)

http://www.anandtech.com/show/8376/intel-disables-tsx-instru...
3 comments
htns 3465 days ago
I wonder what exactly was the bug? As far as I can google it's not been made public. https://www-ssl.intel.com/content/dam/www/public/us/en/docum... (errata for the CPU I have, and I just tested and the TSX instructions aren't disabled on it btw. I probably enabled TSX in BIOS and forgot about it) mentions minor issues with string instructions' interaction with TSX, rdrand having a chance of hanging if called within a transaction, but the bug blamed for the disabling of TSX (HSW136) is just described as "unpredictable system behavior" "under a complex set of internal timing conditions and system events".
link
greglindahl 3465 days ago
If you have a more-recent-than-2014 kernel, BIOS, or stepping, the feature bit ought to be accurate.

So sure, there are some systems in the wild that are broken, but probably not that many.

link
0x0 3465 days ago
Let's say you're deploying to a random cloud VM that may or may not have the latest microcode/BIOS. How do you know if TSX is safe to use? Can it be determined in software by looking at CPUID values? (If so, do all TSX-using libraries/compilers insert such checks?)

The risk of subtle locking bugs in multi threaded applications due to CPU bugs makes me want to shy away from the entire feature.

link
wtallis 3465 days ago
CPUID values would be sufficient. TSX should be correct on Haswell-EX (Xeon E7), Broadwell except for the tablet SoCs (Core M), and all Skaylake, Kaby Lake and newer.
link
greglindahl 3464 days ago
Note that most Linux distros put the latest microcode updates into all of their kernels for any supported version. That means that an updated box with an "old" distro is still going to be OK.
link
0x0 3464 days ago
Does that work under a hypervisor/xen/VM/whatever? Can you apply a microcode update only within a given VM?
link
loeg 3465 days ago
TSX was broken on Haswell CPUs. I don't know which specific newer microarchitecture fixes TSX. Microcode updates have disabled TSX on Haswell for a long time.
link
bonzini 3465 days ago
Haswell Xeon E7 do not have the bug and they do enable TSC.
link