KVM: VMX: Only save/restore MSR_K6_STAR if necessary
Intel hosts only support syscall/sysret in long more (and only if efer.sce
is enabled), so only reload the related MSR_K6_STAR if the guest will
actually be able to use it.
This reduces vmexit cost by about 500 cycles (6400 -> 5870) on my setup.
Signed-off-by: NAvi Kivity <avi@qumranet.com>
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