Implement Thread-Local Storage Variant I

[PandaZ3D]

Thread Local Storage (TLS) has two variants and that choice is architecture specific. A summary of TLS can be found here. x86-64 uses variant II for backwards compatibility, and aarch64 uses variant I. Given this difference, we cannot assume a particular variant when initializing TLS (processor::init_tls). We move TLS initialization out of the processor module and into the image, where both variants are implemented. The choice of which variant to choose is done in an architecture specific image module.

Summary

• Change kernel to use TPDIR_EL1 when compiled for aarch64.
• Move around sections in linker script (aarch64) so that TLS sections show up.
• Implement variant I of TLS (used by aarch64)
• Change TLS initialization to be arch specific

[dbittman]

Can you add a basic kernel test for TLS? Just a test case to make sure it's working, something like having a pre-defined TLS variable with a non-zero magic number, and then testing that loading that value is correct, doesn't have to be crazy.

[PandaZ3D]

aarch64 passes the test, but TLS is not being initialized properly for x86-64. Essentially the offset from the initial TLS allocation area is wrong. The reason why TLS seemed like it worked before was because all initial values of TLS variables were zero. The workaround is to set to the size of the TLS block, plus padding. With this fix, it passes the test.

-  let tcb_base = VirtAddr::from_ptr(tls).offset(full_tls_size).unwrap();
+  let tcb_base = VirtAddr::from_ptr(tls).offset(tls_size).unwrap();

[dbittman]

Great, I suspected there was a bug there!