On Fri, 2007-10-12 at 15:02 -0500, Anthony Liguori wrote: > Hollis Blanchard wrote: > > On Fri, 2007-10-12 at 13:08 -0300, Glauber de Oliveira Costa wrote: > > > >> +config KVM_CLOCK > >> + bool "KVM paravirtualized clock" > >> + depends on PARAVIRT && GENERIC_CLOCKEVENTS > >> + help > >> + Turning on this option will allow you to run a paravirtualized > >> clock > >> + when running over the KVM hypervisor. Instead of relying on a PIT > >> + (or probably other) emulation by the underlying device model, > >> the host > >> + provides the guest with timing infrastructure, as time of day, > >> and > >> + timer expiration. > >> > > > > I must have missed earlier discussion on this topic, so I'm left > > wondering... what's the point? What's wrong with PIT (et al) emulation? > > > > There are three separate reasons, that I know of, to have a PV timer. > > 1) the PIT is periodic. a PV timer can offer a one shot timer which > enables dynticks.
Obviously people have figured out how to do dynticks on real x86 hardware, so I don't accept this reason. :) > 2) the TSC would have to be used as a clocksource. You don't know the > frequency which is the first problem with using the TSC but some systems > have a TSC that changes frequencies. A PV time source gives you more > stable clocksource (although as in glommer's patch, when the TSC can be > used, it's better to use it). As I understand it, the TSC is based on CPU frequency, which changes with power management. Architectural bug. However, PV time still doesn't help here: * The TSC is _user_ accessible, so PV time support in the guest kernel doesn't solve the problem. * It looks like external agents can perform out-of-kernel frequency scaling on x86 (at least I see options for it on IBM blades). So there must already exist some mechanism for a kernel to be informed that the TSC frequency has been changed. > 3) a PV clock can support stolen time calculation which there really > isn't a concept of with emulation. This is true, and I know other platforms support this functionality. I think it's mostly useful for process time accounting. Is that actually supported in this patch? -- Hollis Blanchard IBM Linux Technology Center ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel