implement a new context switch

[Freax13]

This pr reimplements the context switch in such a way that we don't need to map any of the bootloader's memory into the kernel's address space (see #239 for more info). This pr also makes the GDT base address configurable instead of identity mapping it into the kernel's address space.

Depends on rust-osdev/x86_64#385
Closes #239

[phil-opp]

Thanks a lot for implementing this! Unfortunately, I don't have much time right now, but I try my best to review this as soon as possible.

[Freax13]

Thanks a lot for implementing this! Unfortunately, I don't have much time right now, but I try my best to review this as soon as possible.

No worries, take your time!

[Freax13]

Just rebased onto the latest next branch.

[phil-opp]

Thanks a lot! I'm really sorry for the delay in reviewing.

Given that this changes a critical part of the boot process, I think we should get #232 merged and released first since many people tested that version already. This change would then become v0.11.1, which would make it easier to compare the behavior and detect possible regressions. Would that be ok for you?

[Freax13]

Sure, sounds good to me!

[Freax13]

Why was this PR closed? Couldn't we just have switched the target branch to main?

[bjorn3]

Given that the delection of the target branch and the closing of this PR happened at the same time I thinl github auto-closed it.

[phil-opp]

Oh wow, looks like a race condition on GitHub's side. Normally, it just changes the base branch in this case (and it did so for #282), but apparently it handled the "branch deleted" action first for this PR.

[phil-opp]

Ok, restoring the branch allowed me to reopen this PR. I now changed the base branch manually.

[Freax13]

Thanks!