I'm not sure exactly when the first 840 EVO firmware which advertised queued trim support (along with SATA 3.1/3.2 support) was released, but I think that if you last updated firmware (or acquired the drive) before October 2014, you're safe.

However, if you don't update your firmware, you'll suffer from significant performance degradation when reading old files: http://www.anandtech.com/show/9196/samsung-releases-second-8...

The old file performance hit is VERY big, I can't overstate the need to upgrade.

I currently can not notice any performance degradation, and I bought the drive in may 2014, with no further updates. (Unless Arch Linux automatically applies firmware updates, but I doubt that)