LVBS: Refactor per-CPU variables

[sangho2]

This PR refactors the LVBS platform's PerCpuVariables, improving its security, safety, and performance.

Changes:

• Seed initial heap to remove the static PER_CPU_VARIABLE_ADDRESSES array. RefCell is no longer needed.
• Heap-allocate PerCpuVariables on demand when each AP firstly enters VTL1.
• Refactor PerCpuVariables struct for asm access and strict interior mutability (Cell and UnsafeCell).
• Add safe accessor methods.
• Clarify BSP/AP distinction.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type PerCpuVariables is no longer Sync, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  type PerCpuVariables is no longer RefUnwindSafe, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27

--- failure derive_trait_impl_removed: built-in derived trait no longer implemented ---

Description:
A public type has stopped deriving one or more traits. This can break downstream code that depends on those types implementing those traits.
        ref: https://doc.rust-lang.org/reference/attributes/derive.html#derive
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/derive_trait_impl_removed.ron

Failed in:
  type PerCpuVariables no longer derives Clone, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  type PerCpuVariables no longer derives Copy, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/function_missing.ron

Failed in:
  function litebox_platform_lvbs::host::per_cpu_variables::with_per_cpu_variables_mut, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:476
  function litebox_platform_lvbs::host::per_cpu_variables::with_per_cpu_variables_asm, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:494

--- failure function_parameter_count_changed: pub fn parameter count changed ---

Description:
A publicly-visible function now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/function_parameter_count_changed.ron

Failed in:
  litebox_platform_lvbs::mshv::hvcall::init now takes 1 parameters instead of 0, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/hvcall.rs:79

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/inherent_method_missing.ron

Failed in:
  PerCpuVariables::hv_vp_assist_page_as_ptr, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:66

--- failure repr_align_added: repr(align) added ---

Description:
repr(align(N)) was added to a type. This changes its alignment and prevents it from being used inside repr(packed) types.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#repr-align-add
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/repr_align_added.ron

Failed in:
  struct PerCpuVariablesAsm in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:253

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_platform_lvbs::host::per_cpu_variables::RefCellWrapper, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:393

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field vtl0_state of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:41
  field vtl0_locked_regs of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:42
  field gdt of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:43
  field tls of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:44
  field vp_index of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/00818da810027be87eb2bcd090d8bd9bbf67f74e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:45

--- failure struct_pub_field_now_doc_hidden: pub struct field is now #[doc(hidden)] ---

Description:
A pub field of a pub struct is now marked #[doc(hidden)] and is no longer part of the public API.
        ref: https://doc.rust-lang.org/rustdoc/write-documentation/the-doc-attribute.html#hidden
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_pub_field_now_doc_hidden.ron

Failed in:
  field PerCpuVariables.vtl0_state in file /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  field PerCpuVariables.vtl0_locked_regs in file /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  field PerCpuVariables.gdt in file /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  field PerCpuVariables.tls in file /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27
  field PerCpuVariables.vp_index in file /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/host/per_cpu_variables.rs:27

--- failure function_parameter_count_changed: pub fn parameter count changed ---

Description:
A publicly-visible function now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/function_parameter_count_changed.ron

Failed in:
  litebox_runner_lvbs::init now takes 1 parameters instead of 0, in /home/runner/work/litebox/litebox/litebox_runner_lvbs/src/lib.rs:99

[wdcui]

LGTM. Thanks!

[wdcui]

For performance reasons, would we need to initialize APs concurrently if there are too many of them?

[sangho2]

We would have two options:

1. Spin-lock the shared boot stack page until a heap-allocated stack is ready (re-introduce c42157c).
2. Pre-allocate temporal stack pages for all possible vCPUs and reclaim them later.

I implemented 1 (c42157c) but not 2 because I hesitated to pre-allocate 100s of pages (e.g., Azure HBv5 instances support up to 368 vCPUs) and then reverted it just because VTL0 currently does not do concurrent AP/VTL1 initialization (effectively no-op). We can re-introduce it for future proofing.