Dan Magenheimer
2009-Oct-02 17:51 UTC
[Xen-devel] [RFC] Correct/fast timestamping in apps under Xen [4 of 4]: Virt-aware apps
============Premise 4: Apps can become "virtualization aware" in that they can access certain information directly from Xen utilizing an OS-independent mechanism. This information includes not only "Am I running on Xen?" but also, for example, "Is TSC reliable on this physical machine?", "Is rdtsc emulated or native on this virtual machine?", "What is the current incarnation number for this virtual machine?", "Is this virtual machine landlocked?", "What are the pvclock parameters for this virtual machine?", etc. ============ While I am proposing this change in the context of correct/fast timestamping, many industry pundits (including very recently Citrix''s Simon Crosby) have observed that this concept is inevitable because, in many case, a hypervisor obviates the need for an operating system. OS advocates may argue that an app should never circumvent the OS as it makes the app non-portable. Different mechanism can be discussed, e.g. Userland hypercalls, userland-hypervisor shared page, and "special" rdmsr instructions. I like the rdmsr because it is easy to implement and useful for passing a small amount of information. Over time more than one mechanism can evolve. Note that this effectively adds an ABI between the app layer and the hypervisor. So some planning should be employed to ensure that the ABI can be easily extensible and backwards- compatible essentially forever. And since this is an app layer ABI, it might make sense to consider the possibility of standardization across all hypervisors too (e.g. VMware, KVM, HyperV). _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel