The problem is, I can not get into fastboot!
I turn it off, hold Vol+Up and plug in USB, but the blue LED appears only for a couple seconds, then goes off and the phone just continues charging.
I installed Android SDK stuff and I couldnt use it to fastboot CM10 boot.img. I tried Flashtool, AND I installed the drivers.
(Note in case it matters (don't think so): at first I downloaded fastboot drivers alone and installed, but after installing Flashtool those may have been newer drivers, I tried to install them too but it said I already have)
Even after installing the drivers my Xperia U showed no signs of working fastboot, and PC still couldn't recognize it in fastboot.
I had problems installing the drivers, as I found out - because of Windows 8.1, but I got through that.
Another question: Does Xperia U change from ST25a to ST25i depending on kernel/ROM or something? (I have ST25a)