Switch from lazy FPU switching to semi-eager FPU switching: track whether
curproc's xstate ("extended state") is loaded in the CPU or not.
- context switch, sendsig(), vmm, and doing CPU crypto in the kernel all
check the flag and, if set, save the old thread's state to the PCB,
clear the flag, and then load the _blank_ state
- when returning to userspace, if the flag is clear then set it and restore
the thread's state

This simpler tracking also fixes the restoring of FPU state after nested
signal handlers.

With this, %cr0's TS flag is never set, the FPU #DNA trap can no
longer happen, and IPIs are no longer necessary for flushing or
syncing FPU state; on the other hand, restoring xstate while returning
to userspace means we have to handle xrstor faulting if we could
be loading an altered state. If that happens, reset the state,
fake a #GP fault (SIGBUS), and recheck for ASTs.

While here, regularize fxsave/fxrstor vs xsave/xrstor handling, by
using codepatching to switch to xsave/xrstor when present in the
CPU. In addition, code patch in use of xsaveopt in most places
when the CPU supports that. Use the 64bit-wide variants of the
instructions in all cases so that x87 instruction fault IPs are
reported correctly.

This change has three motivations:
1) with modern clang, SSE registers are used even in rcrt0.o, making
lazy FPU switching a smaller benefit vs trap costs
2) the Intel SDM warns that lazy FPU switching may increase power costs
3) post-Spectre rumors suggest that the %cr0 TS flag might not block
speculation, permitting leaking of information about FPU state
(AES keys?) across protection boundaries.

tested by many in snaps; prodding from deraadt@

branches: 1.16.2; 1.16.4;
Fix a typo relating to savefpu alignment

ok kettenis

Move the setting of pcb_fsbase out of reset_segs() into its callers

ok mpi@

Do lazy update/reset of the FS.base and %[def]s segment registers: reseting
segment registers in cpu_switchto if the old thread had made it to userspace
and restoring FS.base only on first return to userspace since context switch.

ok mlarkin@

emul_native is only used for kernel threads which can't dump core, so
delete coredump_trad(), uvm_coredump(), cpu_coredump(), struct md_coredump,
and various #includes that are superfluous.

This leaves compat_linux processes without a coredump callback. If that
ability is desired, someone should update it to use coredump_elf32() and
verify the results...

ok kettenis@

remove irrelevant comments

Unrevert the FS.base diff: the issues were actually elsewhere
Additional testing by jasper@ and pea@

Revert bulk of the FS.base diff, as it causes issues on some machines
and the problem isn't obvious yet.

Add support for per-rthread base-offset for the %fs selector on amd64.
Add pcb_fsbase to the PCB for tracking what the value for the thread
is, and ci_cur_fsbase to struct cpu_info for tracking the CPU's current
value for FS.base, then on return to user-space, skip the setting if the
CPU has the right value already. Non-threaded processes without TLS leave
FS.base zero, which can be conveniently optimized: setting %fs zeros
FS.base for fewer cycles than wrmsr.

ok kettenis@

Normalize sentinel. Use _MACHINE_*_H_ and _<ARCH>_*_H_ properly and consitently.

Discussed and okay drahn@. Okay deraadt@.

Switch from TSS-per-process to TSS-per-CPU, placing the TSS right
next to the cpu's GDT, also making the double-fault stack per-CPU,
leaving it at the top of the page of the CPU's idle process. Inline
pmap_activate() and pmap_deactivate() into the asm cpu_switchto
routine, adding a check for the new pmap already being marked as
active on the CPU. Garbage collect the hasn't-been-used-in-years
GDT update IPI.

Tested by many; ok mikeb@, kettenis@

The LDT is only used by dead compat code now, so load the ldt
register with the null selector (disabling use of it), stop reloading
it on every context switch, and blow away the table itself, as well
as the pcb and pmap bits that were used to track it. Also, delete
two other unused pcb members: pcb_usersp and pcb_flags. (Deleting
pcb_usersp also keeps the pcb_savefpu member aligned properly.)
Finally, delete the defines for the unimplemented AMD64_{GET,SET}_LDT
sysarch() calls.

Tested by various with both AMD and Intel chips
ok mikeb@

First pass at removing clauses 3 and 4 from NetBSD licenses.

Not sure what's more surprising: how long it took for NetBSD to
catch up to the rest of the BSDs (including UCB), or the amount of
code that NetBSD has claimed for itself without attributing to the
actual authors.

OK deraadt@

- remove USER_LDT, it was never in a state where it would copile, nor will
we support i386-compat mode on amd64.

agreed by beck@, dlg@, kettenis@
ok deraadt@, tom@

Missing $OpenBSD$ tags.

Remove the advertising clause in the UCB license which Berkeley
rescinded 22 July 1999.
Checked by ian@, deraadt@ and millert@, arm portion checked by drahn@

an amd64 arch support.
hacked by art@ from netbsd sources and then later debugged
by me into the shape where it can host itself.
no bootloader yet as needs redoing from the
recent advanced i386 sources (anyone? ;)

Fix a typo relating to savefpu alignment

ok kettenis

Move the setting of pcb_fsbase out of reset_segs() into its callers

ok mpi@

Do lazy update/reset of the FS.base and %[def]s segment registers: reseting
segment registers in cpu_switchto if the old thread had made it to userspace
and restoring FS.base only on first return to userspace since context switch.

ok mlarkin@

emul_native is only used for kernel threads which can't dump core, so
delete coredump_trad(), uvm_coredump(), cpu_coredump(), struct md_coredump,
and various #includes that are superfluous.

This leaves compat_linux processes without a coredump callback. If that
ability is desired, someone should update it to use coredump_elf32() and
verify the results...

ok kettenis@

remove irrelevant comments

Unrevert the FS.base diff: the issues were actually elsewhere
Additional testing by jasper@ and pea@

Revert bulk of the FS.base diff, as it causes issues on some machines
and the problem isn't obvious yet.

Add support for per-rthread base-offset for the %fs selector on amd64.
Add pcb_fsbase to the PCB for tracking what the value for the thread
is, and ci_cur_fsbase to struct cpu_info for tracking the CPU's current
value for FS.base, then on return to user-space, skip the setting if the
CPU has the right value already. Non-threaded processes without TLS leave
FS.base zero, which can be conveniently optimized: setting %fs zeros
FS.base for fewer cycles than wrmsr.

ok kettenis@

Normalize sentinel. Use _MACHINE_*_H_ and _<ARCH>_*_H_ properly and consitently.

Discussed and okay drahn@. Okay deraadt@.

Switch from TSS-per-process to TSS-per-CPU, placing the TSS right
next to the cpu's GDT, also making the double-fault stack per-CPU,
leaving it at the top of the page of the CPU's idle process. Inline
pmap_activate() and pmap_deactivate() into the asm cpu_switchto
routine, adding a check for the new pmap already being marked as
active on the CPU. Garbage collect the hasn't-been-used-in-years
GDT update IPI.

Tested by many; ok mikeb@, kettenis@

The LDT is only used by dead compat code now, so load the ldt
register with the null selector (disabling use of it), stop reloading
it on every context switch, and blow away the table itself, as well
as the pcb and pmap bits that were used to track it. Also, delete
two other unused pcb members: pcb_usersp and pcb_flags. (Deleting
pcb_usersp also keeps the pcb_savefpu member aligned properly.)
Finally, delete the defines for the unimplemented AMD64_{GET,SET}_LDT
sysarch() calls.

Tested by various with both AMD and Intel chips
ok mikeb@

First pass at removing clauses 3 and 4 from NetBSD licenses.

Not sure what's more surprising: how long it took for NetBSD to
catch up to the rest of the BSDs (including UCB), or the amount of
code that NetBSD has claimed for itself without attributing to the
actual authors.

OK deraadt@

- remove USER_LDT, it was never in a state where it would copile, nor will
we support i386-compat mode on amd64.

agreed by beck@, dlg@, kettenis@
ok deraadt@, tom@

Missing $OpenBSD$ tags.

Remove the advertising clause in the UCB license which Berkeley
rescinded 22 July 1999.
Checked by ian@, deraadt@ and millert@, arm portion checked by drahn@

an amd64 arch support.
hacked by art@ from netbsd sources and then later debugged
by me into the shape where it can host itself.
no bootloader yet as needs redoing from the
recent advanced i386 sources (anyone? ;)