On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote: > lantianyu1986@xxxxxxxxx writes: > > > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> > > > > Hyper-V guests use the default native_sched_clock() in pv_ops.time.sched_clock > > on x86. But native_sched_clock() directly uses the raw TSC value, which > > can be discontinuous in a Hyper-V VM. Add the generic hv_setup_sched_clock() > > to set the sched clock function appropriately. On x86, this sets > > pv_ops.time.sched_clock to read the Hyper-V reference TSC value that is > > scaled and adjusted to be continuous. > > Hypervisor can, in theory, disable TSC page and then we're forced to use > MSR-based clocksource but using it as sched_clock() can be very slow, > I'm afraid. > > On the other hand, what we have now is probably worse: TSC can, > actually, jump backwards (e.g. on migration) and we're breaking the > requirements for sched_clock(). That (obviously) also breaks the requirements for using TSC as clocksource. IOW, it breaks the entire purpose of having TSC in the first place.
