12-16-2019, 06:26 PM
(12-16-2019, 05:09 AM)danielt Wrote: @Solra Bizna : The hack doesn't seem that nasty when you look at how it works... however when you start to think about the consequences then it does get a bit nastier.
When the kernel boots up it reads the status registers of the online CPUs and decided what workarounds it needs to deploy for the CPUs it finds itself running on. If the A72 cores are not online during the boot then the workarounds are not deployed because they are not needed for hte A53 cores that are online. When we try to online to A72 cores the kernel notices there is a workaround missing and refuses to let them boot (and your desktop will feel slow as a result... it needs the big cores to make things smooth). To get the A72s to work we have to turn off workarounds such as HARDEN_EL2_VECTORS in the kernel. Turning off HARDERN_EL2_VECTORS makes it slightly easier to exploit security bugs in the KVM hyypervisor so it is definitely not a good thing. I'm just about OK turning it off on a laptop but things like this cannot be turned off for generic modular distro kernels so I'm still on the lookout for a proper fix.
Aha. I can definitely see how that would be a problem. This would also explain why only the most recent kernel actually, successfully, onlined the big cores—it has the workarounds disabled.