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:
“It is curious how instinctively one protects the image of oneself from idolatry or any other handling that could make it ridiculous, or too unlike the original to be believed any longer.”
—Virginia Woolf (1882–1941)
“From whichever angle one looks at it, the application of racial theories remains a striking proof of the lowered demands of public opinion upon the purity of critical judgment.”
—Johan Huizinga (1872–1945)
“If we consider what happens in conversation, in reveries, in remorse, in times of passion, in surprises, in the instructions of dreams, wherein often we see ourselves in masquerade,—the droll disguises only magnifying and enhancing a real element, and forcing it on our distinct notice,—we shall catch many hints that will broaden and lighten into knowledge of the secret of nature.”
—Ralph Waldo Emerson (1803–1882)