{lib,arch,plat}: Redo syscall ctx's and swapgs logic

[mogasergiu]

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): [e.g. x86_64 or N/A]
• Platform(s): [e.g. kvm, xen or N/A]
• Application(s): [e.g. app-python3 or N/A]

Additional configuration

Description of changes

To make git bisecting and rebasing significantly easier and avoid
builds breaking across commits, this whole set of changes shall be
introduced under one single all encompassing commit.

Following the introduction of the concept of auxiliary stack pointers,
swapgs, struct uk_syscall_ctx and struct ukarch_sysregs, a number
of things have emerged:

• the aforemenetioned structs are very generic so they should be moved
  under libcontext (arch/)
• swapgs introduces a significant inconsistency between ARM64 and x86_64
  as we never know during an exception the state of
  MSR_GS_BASE/MSR_KERNEL_GS_BASE
• auxiliary stack pointers have increased flexibility as every thread
  and LCPU can have one and have private data stored in there than may
  be accessed anytime, dependency free

Thus, this commit does the following:

1. Move/rename aforementioned structured to libcontext and document them

• lib/syscall_shim/arch/x86_64/sysregs.c -> arch/x86/sysctx.c
• lib/syscall_shim/arch/x86_64include/arch/sysregs.h -> arch/x86/x86_64include/uk/asm/sysctx.h
• s/struct ukarch_sysregs/struct ukarch_sysctx/ (and all related defs)
• struct uk_syscall_ctx from lib/syscall_shim/include/uk/syscall.h to
  include/uk/arch/ctx.h as struct ukarch_execenv
• s/struct uk_syscall_ctx/struct ukarch_execenv/ (and all related defs)
• actually comment these functions
• re-adjust all places that make use of such definitions

2. Get rid of the swapgs, architecture specific holdback by exploiting
   the flexibility of auxiliary stacks through the introduction of a new
   always existing contrl block at their top end:

• introduce struct ukarch_auxspcb under libcontext
• add Unikraft system context as field to it so that we always have and
  know Unikraft TLS (and LCPU in case ox x86_64) in a dependency free
  and assumption free manner
• add a current frame pointer field: since the auxspscb will be part of
  the auxiliary stack, we need to know the safe place where we can start
  using the auxiliary stack area as a stack (this is also helpful in cases
  where we need to nest on the auxstack)
  -for the aforementione fields/structs, init/getter/setter functions have
  been added and documented
• now the swapgs pair will only be done very early during system call
  entry (and only there, not on clone child exit anymore either) just
  enough so that we, first things first, switch to auxstack and push auxsp
  so that on entry to C handler we will know that we must do a call to
  ukarch_sysctx_load on the Unikraft sysctx we can get from the pushed
  auxsp (another benefit of this is we get rid of MSR read/writes)

[skuenzer]

Thanks a lot for your work. As discussed, we will later restructure libcontext and your additions to it with plat rearch and the lcpu library in particular.

Reviewed-by: Simon Kuenzer simon.kuenzer@unikraft.io

@michpappas I am handing over to you.

[michpappas]

With all @razvand's tests passing,

Approved-by: Michalis Pappas michalis@unikraft.io