amd64: move pcb out of kstack to struct thread

[kib]

Summary

This saves 320 bytes of the precious stack space.

The only negative aspect of the change I can think of is that the struct thread increased by 320 bytes obviously, and that 320 bytes are not swapped out anymore. I believe the freed stack space is much more important than that. ALso, current struct thread size is 1392 bytes in size on amd64, so UMA will allocate two thread structures per (4KB) slab, which leaves a space for pcb without increasing zone memory use.

I consider moving the user FPU save area into the dedicated allocation as the next step. Then it might be even possible to reduce the default stack size (not tried yet).

Tested by: pho

[markj]

The only negative aspect of the change I can think of is that the struct thread increased by 320 bytes obviously, and that 320 bytes are not swapped out anymore. I believe the freed stack space is much more important than that.

struct thread is 1392 bytes in size on amd64, so UMA will allocate two thread structures per (4KB) slab. There is thus a lot of space being wasted currently, so I think using some of it for the PCB is fine.

[markj]

I think "top of the stack" is a bit of a misnomer: the base address of a stack is typically called the "bottom" of the stack, but that is confusing here since thread stacks grow down. I don't think md_bottom_of_stack would be any more clear; maybe consider calling it md_stack_base?

sys/amd64/amd64/vm_machdep.c

104 ↗

(On Diff #63624)

Perhaps include the thread pointer in the message?

192 ↗

(On Diff #63624)

The parens are not needed.

359 ↗

(On Diff #63624)

Same as above.

[kib]

Mark' notes.

[alc]

sys/amd64/amd64/machdep.c

1792 ↗

(On Diff #63634)

This sentence is a bit awkward. Do you mean the following? "We initialize the PCB pointer early so that exception handlers will work."

[kib]

Reformulate comment as suggested by alc.