Update docs on libraries cross build (#76614)

* Update docs on libraries cross build

* Apply suggestions from code review

* Changes after review

Co-authored-by: Jan Kotas <jkotas@microsoft.com>

1 files changed, 52 insertions, 88 deletions

diff --git a/docs/workflow/building/libraries/cross-building.md b/docs/workflow/building/libraries/cross-building.md
index 24393ba5cf7..c1f6af6bbf6 100644
--- a/docs/workflow/building/libraries/cross-building.md
+++ b/docs/workflow/building/libraries/cross-building.md
@@ -1,8 +1,7 @@
-Cross Compilation for ARM on Linux
+Cross compile native runtime libraries on Linux
 ==================================
 
-It is possible to build CoreFx on Linux for arm, armel, or arm64 by cross compiling.
-It is very similar to the cross compilation procedure of CoreCLR.
+It is possible to build libraries on Linux for arm, armel, arm64 or other architectures by cross compiling. It is very similar to the cross compilation procedure of CoreCLR.
 
 Requirements
 ------------
@@ -11,7 +10,7 @@ You need a Debian based host, and the following packages need to be installed:
 
     $ sudo apt-get install qemu qemu-user-static binfmt-support debootstrap
 
-In addition, to cross compile CoreFX, the binutils for the target are required. So for arm you need:
+In addition, to cross compile libraries, the binutils for the target are required. So for arm you need:
 
     $ sudo apt-get install binutils-arm-linux-gnueabihf
 
@@ -23,109 +22,74 @@ and for arm64 you need:
 
     $ sudo apt-get install binutils-aarch64-linux-gnu
 
+and similar ones for other architectures.
 
-Generating the rootfs
+Generate the rootfs
 ---------------------
-The `eng/common/cross/build-rootfs.sh` script can be used to download the files needed for cross compilation. It will generate an Ubuntu 16.04 rootfs as this is what CoreFX targets.
+The `eng/common/cross/build-rootfs.sh` script can be used to download the files needed for cross compilation. It can generate rootfs for different operating systems and architectures, see `eng/common/cross/build-rootfs.sh --help` for more details.
 
-    Usage: ./eng/common/cross/build-rootfs.sh [BuildArch] [LinuxCodeName] [lldbx.y] [--skipunmount] --rootfsdir <directory>]
-    BuildArch can be: arm, armel, arm64, x86
-    LinuxCodeName - optional, Code name for Linux, can be: trusty, xenial(default), zesty, bionic, alpine. If BuildArch is armel, LinuxCodeName is jessie(default) or tizen.
-    lldbx.y - optional, LLDB version, can be: lldb3.9(default), lldb4.0, lldb5.0, lldb6.0 no-lldb. Ignored for alpine
+The `build-rootfs.sh` script might need to be launched as root, as it has to make some symlinks to the system. By default it generates the rootfs in `.tools/rootfs/<BuildArch>`, however this can be changed by setting the `ROOTFS_DIR` environment variable or by using `--rootfsdir`.
 
-The `build-rootfs.sh` script must be run as root, as it has to make some symlinks to the system. It will, by default, generate the rootfs in `cross/rootfs/<BuildArch>` however this can be changed by setting the `ROOTFS_DIR` environment variable or by using --rootfsdir.
+For example, to generate an arm Ubuntu 18.04 rootfs:
 
-For example, to generate an arm rootfs:
+    $ ./eng/common/cross/build-rootfs.sh arm bionic
 
-    $ sudo ./eng/common/cross/build-rootfs.sh arm
+And to generate the rootfs elsewhere:
 
-and if you wanted to generate the rootfs elsewhere:
+    $ ./build-rootfs.sh arm bionic --rootfsdir /mnt/rootfs/arm
 
-    $ sudo ./build-rootfs.sh arm --rootfsdir  /mnt/corefx-cross/arm
 
-
-Cross compiling for native CoreFX
+Compile native runtime libraries
 ---------------------------------
-Once the rootfs has been generated, it will be possible to cross compile CoreFX. If `ROOTFS_DIR` was set when generating the rootfs, then it must also be set when running `build.sh`.
-
-So, without `ROOTFS_DIR`:
-
-    $ ./src/Native/build-native.sh debug arm verbose cross
-
-And with:
-
-    $ ROOTFS_DIR=/mnt/corefx-cross/arm ./src/Native/build-native.sh debug arm verbose cross
 
-As usual the generated binaries will be found in `artifacts/bin/TargetOS.BuildArch.BuildType/native` as following:
-
-    $ ls -al ./artifacts/bin/Linux.arm.Debug/native
-    total 988
-    drwxrwxr-x 2 lgs lgs   4096  3  6 18:33 .
-    drwxrwxr-x 3 lgs lgs   4096  3  6 18:33 ..
-    -rw-r--r-- 1 lgs lgs  19797  3  6 18:33 System.IO.Compression.Native.so
-    -rw-r--r-- 1 lgs lgs 428232  3  6 18:33 System.Native.a
-    -rw-r--r-- 1 lgs lgs 228279  3  6 18:33 System.Native.so
-    -rw-r--r-- 1 lgs lgs  53089  3  6 18:33 System.Net.Http.Native.so
-    -rw-r--r-- 1 lgs lgs 266720  3  6 18:33 System.Security.Cryptography.Native.so
-    $ file ./artifacts/bin/Linux.arm.Debug/native/System.Native.so
-    ./bin/Linux.arm.Debug/native/System.Native.so:
-    ELF 32-bit LSB  shared object, ARM, EABI5 version 1 (SYSV),
-    dynamically linked, BuildID[sha1]=fac50f1bd657c1759f0ad6cf5951511ddf252e67, not stripped
-
-
-Compiling for managed CoreFX
+To build native runtime libraries for arm:
+
+    $ ROOTFS_DIR=`pwd`/.tools/rootfs/arm ./build.sh libs.native --cross --arch arm --librariesConfiguration Release
+
+Build artifacts can be found in `artifacts/bin/native/net7.0-<TargetOS>-<BuildArch>-<BuildType>/`:
+
+    $ ls artifacts/bin/native/net7.0-Linux-Release-arm/*
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Globalization.Native.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Globalization.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Globalization.Native.so.dbg
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Compression.Native.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Compression.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Compression.Native.so.dbg
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Ports.Native.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Ports.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.IO.Ports.Native.so.dbg
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Native.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Native.so.dbg
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Net.Security.Native.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Net.Security.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Net.Security.Native.so.dbg
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Security.Cryptography.Native.OpenSsl.a
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Security.Cryptography.Native.OpenSsl.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Security.Cryptography.Native.OpenSsl.so.dbg
+
+    $ file artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Native.so
+    artifacts/bin/native/net7.0-Linux-Release-arm/libSystem.Native.so: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, BuildID[sha1]=5f6f6f9c4012dffed133624867adf32ac2af130d, stripped
+
+
+Compile managed runtime libraries on Linux
 ============================
-The managed components of CoreFX are architecture-independent and thus do not require a special build for arm, armel or arm64.
+The managed components of libraries are architecture-independent and, thus, do not require a special build for arm, armel, arm64 or other architectures (this is true if ILLinker trimming is disabled with `/p:ILLinkTrimAssembly=false`).
 
 Many of the managed binaries are also OS-independent, e.g. System.Linq.dll, while some are OS-specific, e.g. System.IO.FileSystem.dll, with different builds for Windows and Linux.
 
-    $ ROOTFS_DIR=/mnt/corefx-cross/arm ./build.sh --arch arm
-
-You can also build just managed code with:
-
-    $ ./build.sh --arch arm /p:BuildNative=false
-
-The output is at `artifacts/bin/[BuildSettings]` where `BuildSettings` looks something like `net5.0-<TargetOS>-Debug-<Architecture>`. Ex: `artifacts/bin/net5.0-Linux-Debug-x64`. For more details on the build configurations see [project-guidelines](/docs/coding-guidelines/project-guidelines.md)
-
-Building corefx for Linux ARM Emulator
-=======================================
-
-It is possible to build corefx binaries (native and managed) for the Linux ARM Emulator (latest version provided here: [#5394](https://github.com/dotnet/runtime/issues/5394)).
-The `scripts/arm32_ci_script.sh` script does this.
-
-The following instructions assume that:
-* You have set up the extracted emulator at `/opt/linux-arm-emulator` (such that `/opt/linux-arm-emulator/platform/rootfs-t30.ext4` exists)
-* The mount path for the emulator rootfs is `/opt/linux-arm-emulator-root` (change this path if you have a working directory at this path).
-
-All the following instructions are for the Release mode. Change the commands and files accordingly for the Debug mode.
-
-To just build the native and managed corefx binaries for the Linux ARM Emulator, run the following command:
-```
-prajwal@ubuntu ~/corefx $ ./scripts/arm32_ci_script.sh \
-    --emulatorPath=/opt/linux-arm-emulator \
-    --mountPath=/opt/linux-arm-emulator-root \
-    --buildConfig=Release
-```
-
-The Linux ARM Emulator is based on the soft floating point and thus the native binaries are generated for the armel architecture. The corefx binaries generated by the above command can be found at `~/corefx/artifacts/bin/Linux.armel.Release`, `~/corefx/artifacts/bin/Linux.AnyCPU.Release`, `~/corefx/artifacts/bin/Unix.AnyCPU.Release`, and `~/corefx/artifacts/bin/AnyOS.AnyCPU.Release`.
-
+Build of managed runtime libraries requires presence of built native runtime libraries.
 
-Build corefx for a new architecture
-===================================
+To build managed runtime libraries for arm (architecture-dependent, can't be used on other architectures):
 
-When building for a new architecture you will need to build the native pieces separate from the managed pieces in order to correctly boot strap the native runtime. Instead of calling build.sh directly you should instead split the calls like such:
+    $ ./build.sh libs.sfx --arch arm --librariesConfiguration Release
 
-Example building for armel
-```
-src/Native/build-native.sh armel
---> Output goes to artifacts/bin/runtime/net5.0-Linux-Debug-armel
+Note that by default ILLinker trimming is enabled and libraries built above for arm can't be used on other arches. To build architecture-independent managed runtime libraries for arm:
 
-build /p:TargetArchitecture=x64 /p:BuildNative=false
---> Output goes to artifacts/bin/runtime/net5.0-Linux-Debug-x64
-```
+    $ ./build.sh libs.sfx --arch arm --librariesConfiguration Release /p:ILLinkTrimAssembly=false
 
-The reason you need to build the managed portion for x64 is because it depends on runtime packages for the new architecture which don't exist yet so we use another existing architecture such as x64 as a proxy for building the managed binaries.
+Build artifacts can be found in `artifacts/bin/microsoft.netcore.app.runtime.<TargetOS>-<BuildArch>/<BuildType>/runtimes/<TargetOS>-<BuildArch>/lib/net7.0/`. For more details on the build configurations see [project-guidelines](/docs/coding-guidelines/project-guidelines.md).
 
-Similar if you want to try and run tests you will have to copy the managed assemblies from the proxy directory (i.e. `net5.0-Linux-Debug-x64`) to the new architecture directory (i.e `net5.0-Linux-Debug-armel`) and run code via another host such as corerun because dotnet is at a higher level and most likely doesn't exist for the new architecture yet.
+Both native and managed runtime libraries can be built at the same time with:
 
-Once all the necessary builds are setup and packages are published the splitting of the build and manual creation of the runtime should no longer be necessary.
+    $ ROOTFS_DIR=`pwd`/.tools/rootfs/arm ./build.sh --cross --arch arm --librariesConfiguration Release --subset libs.native+libs.sfx