Re: [PATCH] lguest: disable SYSENTER for guests

[Avi Kivity <avi@qumranet.com>]

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.