Rusty Russell wrote: > The SYSENTER instruction jumps to a pre-programmed address at > privilege level 0. We must not allow execution of guest code at that > privilege level, so disable sysenter when we enter the guest (and > re-enable it on return). This fixes current case where guest > userspace can crash host. > > This save/restore adds 3% to guest context switch times. (If only > there were some kind of scheduler hook or something which would tell > us when we were being preempted so we could fix this up lazily. But > what kind of daredevil coder would propose such a thing?) > > Ah, so this is why you want ->next in preempt hooks. Well, my plan for this sort of thing (for kvm has the same issues with the *STAR family of msrs) is to add a new hook on switching from kernel to userspace, and swap those msrs there. This allows not only the guest1->guest2 case to be optimized, but also guest->kthread->guest, which is a common pattern with I/O (and very common with -rt, which runs interrupts in threads). -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
