runtime: clean up system call code

[prattmic]

The code used to make system calls from the runtime is a mess and a big pain point when making OS-level changes in the runtime.

Quick summary of some of the pain points:

• Completely independent implementation from package syscall.
• System call stubs exist in a variety of OS- and arch-specific sys* files (57, to be precise, mostly assembly), totaling ~23k lines of code [1], which is 100% hand-written. Adding a new syscall stub often requires adding new assembly to 10+ files.
• There is no generic Syscall function that can be used to make an arbitrary system call. You must add a new stub to make a new call.

Some things we could do to improve the situation:

• Add generic Syscall functions, used by other stubs. And/or use code generation to generate stubs rather than doing so manually.
• Move syscall code to a new package (like runtime/internal/syscall or perhaps runtime/internal/linux?) to avoid littering the runtime package itself with so many files.
• Make package syscall depend on the runtime's Syscall stub so we only have a single implementation.

[1] Some of this code is things other than syscall stubs, like thread entrypoints.

cc @aclements @mknyszek @cherrymui

[aclements]

And/or use code generation to generate stubs rather than doing so manually.

All or most of the information we would need to generate the stubs is already in the syscall package. The needs of the syscall package are of course a bit different, but the metadata should be effectively the same.

Orthogonal to code generation, given a generic runtime Syscall function (or a few of them for different numbers of arguments), would there even be any overhead to having the stubs be in Go rather than assembly? In addition to significantly reducing the amount of assembly, I think this would encourage us to do better error handling.

[cherrymui]

Just to add another pain point: the stubs handle errors differently. Some returns errno (negated or not?), some just crash hard (writing to an invalid address), which is difficult to debug sometimes.

[gopherbot]

Change https://go.dev/cl/383999 mentions this issue: runtime/internal/syscall: new package for linux

[ii64]

Seems this introduce build error, i got:
gofrontend commit 45fd14ab8baf5e86012a808426f8ef52c1d77943

[cherrymui]

@ii64 this issue is about a plan for the future, not what has happened. Your comment is unrelated to this issue.

Looks like you are using gollvm. What version of gollvm are you using? Try updating to the latest. If that still fail, please file a new issue. Thanks.

[gopherbot]

Change https://go.dev/cl/388094 mentions this issue: test: rename live_syscall.go to live_uintptrkeepalive.go

[gopherbot]

Change https://go.dev/cl/388477 mentions this issue: syscall: define Syscall in terms of RawSyscall

[gopherbot]

Change https://go.dev/cl/388475 mentions this issue: runtime/internal/syscall, syscall: replace RawSyscall6 with runtime implementation