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With PUSH2/POP2, it's "only" 8 more instructions > on each side. > > If the overhead is in the noise, I'd be very strongly inclined to say KVM should > swap at entry/exit regardless of kernel behavior so that we don't have to special > case accesses on the back end. I tried raising this point at plumbers but I don't think it came through well. You can't unconditionally use PUSH2/POP2 in the VMExit, because at that point in time it's the guest's XCR0 in context.  If the guest has APX disabled, PUSH2 in the VMExit path will #UD. You either need two VMExit handlers, one APX and one non-APX and choose based on the guest XCR0 value, or you need a branch prior to regaining speculative safety, or you need to save/restore XCR0 as the first action.  It's horrible any way you look at it. I've asked both Intel and AMD for changes to VT-x/SVM to have a proper host/guest split of XCR0 which hardware manages on entry/exit.  It's the only viable option in my opinion, but it's still an unknown period of time away and not going to exist in the first APX-capable hardware. ~Andrew