With virtual machines, the host kernel runs in privileged mode to create

a virtual machine that runs in user mode. The virtual machine provides

the illusion that the guest kernel runs on its own machine in privileged

mode, even though it is actually running in user mode.

Early versions of the x86 architecture (pre-2006) were not completely

virtualizablethese systems could not guarantee to run unmodified

guest operating systems properly. One problem was the popf pop flags

instruction that restores the processor status word. When popf was run

in privileged mode, it changed both the ALU flags (e.g., the condition

codes) and the systems flags (e.g., the interrupt mask). When popf was

run in unprivileged mode, it changed just the ALU flags.

a) Why do instructions like popf prevent transparent virtualization of

the (old) x86 architecture?

b) How would you change the (old) x86 hardware to fix this problem?