amd64: Add sysctl knobs for loader tunables

[zlei]

Summary

The following loader tunable do not have corresponding sysctl MIB
entries. Add them so that they can be retrieved, and sysctl -T will
also report them correctly.

1. hw.clflush_disable
2. hw.memtest.tests
3. hw.skz63_enable
4. hw.use_xsave
5. hw.vmm.ept.use_exec_only
6. hw.vmm.ept.use_hw_ad_bits
7. hw.vmm.ept.use_superpages
8. hw.vmm.npt.enable_superpage
9. hw.xsave_mask

MFC after: 1 week

[alc]

sys/amd64/vmm/amd/npt.c

51

62

I would suggest initializing the variables at their definitions, rather than at run-time.

[kib]

sys/amd64/amd64/fpu.c

173

The description is wrong.

That said, I do not see much use for these and other x86-related tunables to be exposed as sysctl. They are debugging tools, useful only for people who understand the code.

[zlei]

sys/amd64/amd64/fpu.c

173

Thanks for the explanation.
That make sense. Those tunables are not documented and confused me until git blame shows theirs history.

[kib]

sys/amd64/amd64/fpu.c

173

If you want to add some information about these tunables, a source comment would be appropriate.

[zlei]

Abandoned as no need to add sysctl knobs for debugging purpose only tunables.

They are debugging tools, useful only for people who understand the code.

sys/amd64/vmm/amd/npt.c

66

Actually the flag enable_superpage can be retrieved from npt_flags which is exposed via sysctl MIB hw.vmm.npt.pmap_flags.

So hw.vmm.npt.enable_superpage should be for debugging purpose only. A new sysctl knob for it is redundant.

sys/amd64/vmm/intel/ept.c

76

Redundant.
They can be retrieved from sysctl MIB hw.vmm.ept.pmap_flags.