SVM: vmload/vmsave-free VM exits?

[Jan Kiszka <jan.kiszka@web.de>]

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

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