Hi, studying the VM exit logic of Jailhouse, I was wondering when AMD's vmload/vmsave can be avoided. Jailhouse as well as KVM currently use these instructions unconditionally. However, I think both only need GS.base, i.e. the per-cpu base address, to be saved and restored if no user space exit or no CPU migration is involved (both is always true for Jailhouse). Xen avoids vmload/vmsave on lightweight exits but it also still uses rsp-based per-cpu variables. So the question boils down to what is generally faster: A) vmload vmrun vmsave B) wrmsrl(MSR_GS_BASE, guest_gs_base) vmrun rdmsrl(MSR_GS_BASE, guest_gs_base) Of course, KVM also has to take into account that heavyweight exits still require vmload/vmsave, thus become more expensive with B) due to the additional MSR accesses. Any thoughts or results of previous experiments? Jan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
