The paravirt_ops mechanism provides a way for the Linux kernel, when
running in a virtualized mode, to hook efficiently into the hypervisor for
privileged operations. Over time, processors have grown hardware features
aimed at supporting virtualization, but there has still been a performance
benefit to implementing some operations through paravirt_ops. That
situation would appear to be changing, though.
VMI is a paravirtualization layer for VMWare, built on top of
paravirt_ops. Recently, developers at VMWare ran a series of tests and came to an
interesting conclusion: with contemporary hardware, using VMI did not
improve the performance of guest systems. Indeed, it made things worse.
Reasonable hardware virtualization should be available on almost all
systems that matter in the near future, so VMWare's developers have decided
that VMI no longer makes sense; they are now planning to remove it.
KVM developer Avi Kivity noted that a
similar conclusion had been reached in that camp; KVM will be dropping
support for some paravirtualized operations in the near future. That
leaves two other systems - Xen and lguest - using paravirt_ops. Xen, it
seems, will continue to do so for some time, and lguest is highly unlikely
to ever sacrifice sufficient puppies to move to hardware virtualization.
So paravirt_ops will remain for a little while yet, but the its eventual
demise would appear to be in the cards. When it goes, it may just take
lguest with it.
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