On Wed, 2007-03-07 at 01:17 -0800, Zachary Amsden wrote: > Ingo Molnar wrote: > > For example, VMI_CALL_SetAlarm takes a 'cycles' argument. Cycles is a > > quite bad unit for an API, it should be absolute time, nanosec or > > picosec based instead. We could easily see CPUs that have /no concept of > > > > Actually, putting the unit in terms of cycles is more portable and > flexible. Rather than perform a conversion from cycles to > nano/femto/pico seconds, the raw cycle count is exposed, along with the > current clock frequency. This allows the timer infrastructure to merely > do one conversion, from cycles to real time, rather than converting to > an arbitrary time unit which may change with operating systems and time > and thus break the ABI. Putting the unit in terms of cycles is just ugly. Virtual hardware should provide the easiest interface and in case of time this _IS_ nanoseconds. nanoseconds is neither an arbitrary time unit nor will it change anytime soon to femtoseconds. So your argument that the ABI might break is just a strawman. The cycles conversion gets ugly as hell, as you want to have absolute time for your clock event reprogramming. This requires 128 bit math in the reprogramming path for nothing and I'm not going to put it there. Even worse on a Linux host we would convert ktime_t to some virtual hardware clock on the guest side, feed it through paravirt to the host and convert it back to ktime_t as the host uses an hrtimer to schedule the next guest event. The whole rush of paravirt ops leads to an arbitrary number of virtual clock source and clock event devices instead of having one virtual silicon with a sane design and a per hypervisor backend. Paravirtualized kernels should provide _sane_ silicon emulations rather than dumping more crap on the kernel developers by competing with the real silicon vendors for the BDHA (Brain Damaged Hardware Award). tglx _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxx https://lists.osdl.org/mailman/listinfo/virtualization
