Handling Critical Instructions
The conditions for ISA virtualization expressed in Theorem 1 may be relaxed at the expense of the efficiency property. VMMs for non-virtualizable ISAs (in the Popek and Goldberg's sense) have routinely been built.
The virtualization of such architectures requires correct handling of critical instructions, i.e., sensitive but unprivileged instructions. One approach, known as patching, adopts techniques commonly used in dynamic recompilation: critical instructions are discovered at run-time and replaced with a trap into the VMM. Various mechanisms, such as the caching of emulation code or hardware assists, have been proposed to make the patching process more efficient. A different approach is that of paravirtualization, which requires guest operating systems to be modified (ported) before running in the virtual environment.
Read more about this topic: Popek And Goldberg Virtualization Requirements
Famous quotes containing the words handling, critical and/or instructions:
“Many more children observe attitudes, values and ways different from or in conflict with those of their families, social networks, and institutions. Yet today’s young people are no more mature or capable of handling the increased conflicting and often stimulating information they receive than were young people of the past, who received the information and had more adult control of and advice about the information they did receive.”
—James P. Comer (20th century)
“Somewhere it is written that parents who are critical of other people’s children and publicly admit they can do better are asking for it.”
—Erma Bombeck (20th century)
“They had supposed their formula was fixed.
They had obeyed instructions to devise
A type of cold, a type of hooded gaze.
But when the Negroes came they were perplexed.
These Negroes looked like men....”
—Gwendolyn Brooks (b. 1917)