Add support for ARCH=um

[nathanchance]

Now that ARCH=um builds with clang, let's make sure it stays working!

NOTE: This PR depends on this PR for mainline; once it is merged, this one can be too.

[nickdesaulniers]

if you remove the closes tag, then in a follow up, we can work on boot testing this?

[nickdesaulniers]

Probably want to update some docs. Homepage and kernel docs?

[nathanchance]

if you remove the closes tag, then in a follow up, we can work on boot testing this?

Sure, I can remove the closes tag. I don't think that we can boot test this with our current flow, as UML is a virtual machine within itself, so we would not use QEMU for testing. I'll look into that tomorrow.

[nickdesaulniers]

we can work on boot testing this?

I don't think that we can boot test this with our current flow, as UML is a virtual machine within itself, so we would not use QEMU for testing. I'll look into that tomorrow.

Right, I'm pretty sure it's just an ELF executable. You just run the image and pass it command line parameters.

[nathanchance]

Yup, I have this working locally:

$ tuxmake -a um -C $CBL_SRC/linux-next -k defconfig -r podman -t clang-nightly LLVM=1 kernel
...
I: config: PASS in 0:00:02.285515
I: default: PASS in 0:00:15.879420
I: kernel: PASS in 0:00:04.387773
I: build output in /home/nathan/.cache/tuxmake/builds/56

# sudo is critical here, the root_fs needs to be owned by root to work properly
$ sudo sh -c "mkdir root_fs && zstd -c -d $CBL_GIT/boot-utils/images/x86_64/rootfs.cpio.zst | cpio -D root_fs -di"
...

$ $HOME/.cache/tuxmake/builds/56/linux rootfstype=hostfs rootflags=$PWD/root_fs
Core dump limits :
        soft - NONE
        hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking environment variables for a tempdir...none found
Checking if /dev/shm is on tmpfs...OK
Checking PROT_EXEC mmap in /dev/shm...OK
Adding 25071616 bytes to physical memory to account for exec-shield gap
Linux version 5.17.0-rc6-00019-g7c23c8af7393-dirty (tuxmake@tuxmake) (Debian clang version 15.0.0-++20220321071824+df4814d45d6b-1~exp1~20220321071914.194, Debian LLD 15.0.0) #1 @1647970143
Zone ranges:
  Normal   [mem 0x0000000000000000-0x00000000637e8fff]
Movable zone start for each node
Early memory node ranges
  node   0: [mem 0x0000000000000000-0x00000000037e8fff]
Initmem setup node 0 [mem 0x0000000000000000-0x00000000037e8fff]
Built 1 zonelists, mobility grouping on.  Total pages: 14117
Kernel command line: rootfstype=hostfs rootflags=/home/nathan/tmp/root_fs root=98:0 console=tty
...
Run /sbin/init as init process
Starting syslogd: OK
Starting klogd: OK
Running sysctl: OK
Saving random seed: SKIP (read-only file system detected)
Starting network: OK
Linux version 5.17.0-rc6-00019-g7c23c8af7393-dirty (tuxmake@tuxmake) (Debian clang version 15.0.0-++20220321071824+df4814d45d6b-1~exp1~20220321071914.194, Debian LLD 15.0.0) #1 @1647970143
Linux version 5.17.0-rc6-00019-g7c23c8af7393-dirty (tuxmake@tuxmake) (Debian clang version 15.0.0-++20220321071824+df4814d45d6b-1~exp1~20220321071914.194, Debian LLD 15.0.0) #1 @1647970143
Stopping network: OK
Saving random seed: SKIP (read-only file system detected)
Stopping klogd: OK
Stopping syslogd: OK
The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL to all processes
Requesting system poweroff
reboot: System halted

I will wire this up into boot-utils shortly as boot-uml.sh, then I will update this PR.

[nathanchance]

Alright, this now depends on ClangBuiltLinux/boot-utils#59. Once that is good to go, I'll bump boot-utils here, then this pull request should be mergeable!

[nathanchance]

Alright, boot-utils has been updated and I have added a patch for a next regression. This should be mergeable, please let me know what you think.

[nickdesaulniers]

nice work! consider dropping all the intermediary regen patches that just churn the dotfiles. 🛡️ 🚢

[nathanchance]

Thanks for the review as always!