Add vDSO support for timekeeping devices that support the KVM/XEN
paravirtual clock API.
Also, expose, in the userspace-accessible '<machine/pvclock.h>',
definitions that will be needed by 'libc' to support
'VDSO_TH_ALGO_X86_PVCLK'.
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31418
Consolidate more hypervisor-agnostic functionality behind a new 'struct
pvclock' API.
This should also make it easier to subsequently add hypervisor-agnostic
vDSO timekeeping support.
Also, perform some clean-up:
- Remove 'pvclock_get_last_cycles()'; do not allow external access
to 'pvclock_last_systime' since this is not necessary.
- Consolidate/simplify wall and system time reading codepaths.
- Ensure correct ordering within wall and system time reading
codepaths via 'atomic(9)' and 'rdtsc_ordered()' rather than via
'rmb()'.
- Remove some extra newlines.
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31418
This can later use this to determine the TSC frequency like is done with
VMware, instead of using a DELAY loop that is not always accurate in an VM.
MFC after: 1 month
KVM clock shares the same data structures between the guest and the host
as Xen so it makes sense to just have a single copy of this code.
Differential Revision: https://reviews.freebsd.org/D1429
Reviewed by: royger (eariler version)
MFC after: 1 month
