Win 7 Wrong number of cores? Slow?

[KwukDuck]

After updating to 4.3.0 from 4.2.18 yesterday i encountered a lot of problems, half of my linux VM's were not able to boot, the VM crashed as soon as it hit GRUB. After messing around half a day i migrated the installations to new installations that did work for some reason, i'm guessing something changed with the vdi format but i can't really pin it down.

Anyway, i noticed i got an error for most of my VM's as they are set to use all 8 cores of my AMD FX-8350, This always worked fine with an execution cap set to prevent the VM's from locking up the host. But now VBox tells me my host only has 4 cores and using more will cause performance loss. So i set it back to 4 and all worked, but noticably slowed than it used to run. (note that there's still 8 cores shown in the slider bare as 'green'). As soon as i set anything over 4 cores both the guest and host system run into problems, both become horribly slow, like pretty much unresponsive and the guest (at least in linux) gives ' CPU# soft lockup for xxx seconds '.

Did something change in the way the cores are addressed? Can i get my old performance back with some settings?

Thanks in advance.

[Perryg]

VirtualBox shows cores and hyperthreads. 4 cores with hyperthreads will show as 8 cores.
Rule of thumb is you allocate only cores and should leave one for the host. So 4 cores would be no more than 3 to the guest.

Before 4.3 there were a lot of people over committing the cores thinking the threads were cores and as a result they had issues, some of which were sever. Now you are notified if you try to go over the actual cores unless you really want to.