Mention red zone in APE ABI specification

Author: jart

ape/specification.md

@@ -306,6 +306,19 @@ APE binaries use the System V ABI, as defined by:
 
 There are however a few changes we've had to make.
 
+### No Red Zone
+
+Actually Portable Executables that have Windows and/or bare metal in
+their support vector MUST be compiled using `-mno-red-zone`. This is
+because, on Windows, DLLs and other software lurking in the va-space
+might use tricks like SetThreadContext() to take control of a thread
+whereas on bare metal, it's also generally accepted that kernel-mode
+code cannot assume a red zone either due to hardware interrutps that
+pull the exact same kinds of stunts.
+
+APE software that only has truly System V ABI conformant OSes (e.g.
+Linux) in their support vector MAY use the red zone optimization.
+
 ### Thread Local Storage
 
 #### aarch64
@@ -668,6 +681,11 @@ converged on a strong consensus of having a page size that's 4096 bytes.
 However this convention was never guaranteed. New computers have become
 extremely popular, such as Apple Silicon, that use a 16kb page size.
 
+By convention, Cosmopolitan Libc currently generates ELF headers for
+x86-64 that are strictly aligned on a 4096-byte page size. On ARM64
+Cosmopolitan is currently implemented to always generate ELF headers
+aligned on a 16kb page size.
+
 In addition to being page size agnostic, APE software that cares about
 working correctly on Windows needs to be aware of the concept of
 allocation granularity. While the page size on Windows is generally 4kb