diff --git a/Directory.Build.props b/Directory.Build.props
index 3014b766801b9..be25c186455fe 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -2,4 +2,9 @@
 <Project>
   <Import Project="dir.common.props" />
   <Import Project="Sdk.props" Sdk="Microsoft.DotNet.Arcade.Sdk" />
+
+  <PropertyGroup>
+    <!-- we're not building all of llvm for now in source build, just objwriter -->
+    <BuildObjWriterOnly Condition="'$(BuildObjWriterOnly)' == '' and '$(DotNetBuildFromSource)' == 'true'">true</BuildObjWriterOnly>
+  </PropertyGroup>
 </Project>
diff --git a/dir.common.props b/dir.common.props
index 84f76330bf345..ba2d7e7b7dc28 100644
--- a/dir.common.props
+++ b/dir.common.props
@@ -72,6 +72,11 @@
     <PreReleaseLabel>preview8</PreReleaseLabel>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <!-- llvm-project doesn't support Arcade-driven target framework filtering. -->
+    <NoTargetFrameworkFiltering>true</NoTargetFrameworkFiltering>
+  </PropertyGroup>
+
   <!-- Set up common target properties that we use to conditionally include sources -->
   <PropertyGroup>
     <TargetsFreeBSD Condition="'$(BuildOS)' == 'FreeBSD'">true</TargetsFreeBSD>
diff --git a/eng/SourceBuild.props b/eng/SourceBuild.props
new file mode 100644
index 0000000000000..f99e6f25c8271
--- /dev/null
+++ b/eng/SourceBuild.props
@@ -0,0 +1,7 @@
+<Project>
+
+  <PropertyGroup>
+    <GitHubRepositoryName>llvm-project</GitHubRepositoryName>
+  </PropertyGroup>
+
+</Project>
diff --git a/eng/SourceBuildPrebuiltBaseline.xml b/eng/SourceBuildPrebuiltBaseline.xml
new file mode 100644
index 0000000000000..b2fb074527888
--- /dev/null
+++ b/eng/SourceBuildPrebuiltBaseline.xml
@@ -0,0 +1,8 @@
+<!-- Whenever altering this or other Source Build files, please include @dotnet/source-build-internal as a reviewer. -->
+<!-- See aka.ms/dotnet/prebuilts for guidance on what pre-builts are and how to eliminate them. -->
+
+<UsageData>
+  <IgnorePatterns>
+    <UsagePattern IdentityGlob="Microsoft.SourceBuild.Intermediate.*" />
+  </IgnorePatterns>
+</UsageData>
diff --git a/eng/Version.Details.xml b/eng/Version.Details.xml
index 847a9b178ce09..63a2a0fe898dc 100644
--- a/eng/Version.Details.xml
+++ b/eng/Version.Details.xml
@@ -1,11 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
+    <Dependency Name="Microsoft.SourceBuild.Intermediate.source-build-reference-packages" Version="8.0.0-alpha.1.23266.3">
+      <Uri>https://github.com/dotnet/source-build-reference-packages</Uri>
+      <Sha>e2e64d25662c00a35726d3c52f969a50edaa4f48</Sha>
+      <SourceBuild RepoName="source-build-reference-packages" ManagedOnly="true" />
+    </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>
     <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="8.0.0-beta.23378.2">
       <Uri>https://github.com/dotnet/arcade</Uri>
       <Sha>54dd37d44a2adfb8b966fac466c2ece40f8b20dd</Sha>
+      <SourceBuild RepoName="arcade" ManagedOnly="true" />
     </Dependency>
     <Dependency Name="Microsoft.DotNet.Helix.Sdk" Version="8.0.0-beta.23378.2">
       <Uri>https://github.com/dotnet/arcade</Uri>
diff --git a/eng/azure-pipelines.yml b/eng/azure-pipelines.yml
index 8ba7ccc1e5f49..885dd6acbbc58 100644
--- a/eng/azure-pipelines.yml
+++ b/eng/azure-pipelines.yml
@@ -27,6 +27,13 @@ stages:
       enablePublishBuildArtifacts: true
       enablePublishBuildAssets: true
       enablePublishUsingPipelines: true
+      enableSourceBuild: true
+      sourceBuildParameters:
+        platforms:
+        - name: 'AlmaLinux_8_Portable'
+          container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:almalinux-8-source-build'
+          jobProperties:
+            timeoutInMinutes: 600
       jobs:
 
       ############ LINUX BUILD ############
diff --git a/llvm.proj b/llvm.proj
index 507c9cec9ff5e..23a2ff3b2140a 100644
--- a/llvm.proj
+++ b/llvm.proj
@@ -40,20 +40,33 @@
     cmake -DCMAKE_INSTALL_DO_STRIP=1 -DCMAKE_INSTALL_PREFIX=$(_LLVMInstallDir) -DCMAKE_INSTALL_COMPONENT=FileCheck -P cmake_install.cmake
     </_CMakeInstallCommand>
     <_CMakeInstallCommand Condition="'$(BuildObjWriterOnly)' != 'true'">$(_SetupEnvironment) cmake -DCMAKE_INSTALL_DO_STRIP=1 -DCMAKE_INSTALL_PREFIX=$(_LLVMInstallDir) -P cmake_install.cmake</_CMakeInstallCommand>
+    <_CrossBuild>false</_CrossBuild>
+    <_CrossBuild Condition="'$(ClangTarget)' != ''">true</_CrossBuild>
+    <_hostArch>$([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant)</_hostArch>
+    <_CrossBuild Condition="'$(BuildOS)' == 'Windows_NT' and '$(TargetArchitecture)' != '$(_hostArch)'">true</_CrossBuild>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(BuildOS)' != 'Windows_NT'">
-    <_CrossCFlags>--target=$(ClangTarget)</_CrossCFlags>
-    <_CrossCFlags Condition="'$(BuildOS)' == 'Linux'">$(_CrossCFlags) --sysroot=$(ROOTFS_DIR) --gcc-toolchain=$(ROOTFS_DIR)/usr</_CrossCFlags>
-    <_SharedLinkerFlags>$(_CrossCFlags)</_SharedLinkerFlags>
+    <_CrossCFlags Condition="'$(_CrossBuild)' == 'true'">--target=$(ClangTarget)</_CrossCFlags>
+    <_CrossCFlags Condition="'$(_CrossBuild)' == 'true' and '$(BuildOS)' == 'Linux'">$(_CrossCFlags) --sysroot=$(ROOTFS_DIR) --gcc-toolchain=$(ROOTFS_DIR)/usr</_CrossCFlags>
+    <_CFlags>$(_CrossCFlags)</_CFlags>
+    <_SharedLinkerFlags>$(_CFlags)</_SharedLinkerFlags>
     <_SharedLinkerFlags Condition="'$(BuildOS)' == 'Linux'">-Wl,--build-id -Wl,-rpath,'%5C%24ORIGIN' -fuse-ld=lld $(_SharedLinkerFlags)</_SharedLinkerFlags>
     <ClangBinDir Condition="'$(ClangBinDir)' == ''">/usr/bin</ClangBinDir>
   </PropertyGroup>
 
-  <ItemGroup>
+  <ItemGroup Condition="'$(_CrossBuild)' == 'true'">
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm64'" Include="-DLLVM_TARGET_ARCH=AARCH64" />
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm'" Include="-DLLVM_TARGET_ARCH=ARM" />
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'x64'" Include="-DLLVM_TARGET_ARCH=X86_64" />
+    <!-- Enable CMake cross-compilation support by setting CMAKE_SYSTEM_NAME manually, see https://github.com/llvm/llvm-project/issues/52819 -->
+    <_LLVMBuildArgs Condition="'$(BuildOS)' == 'Windows_NT'" Include="-DCMAKE_SYSTEM_NAME=Windows" />
+    <_LLVMBuildArgs Condition="'$(BuildOS)' == 'Linux'" Include="-DCMAKE_SYSTEM_NAME=Linux" />
+    <_LLVMBuildArgs Condition="'$(BuildOS)' == 'OSX'" Include="-DCMAKE_SYSTEM_NAME=Darwin" />
+    <_LLVMBuildArgs Condition="'$(BuildOS)' == 'Linux'" Include='-DCROSS_TOOLCHAIN_FLAGS_LLVM_NATIVE="-DCMAKE_EXE_LINKER_FLAGS_INIT=-fuse-ld=lld;-DCMAKE_SHARED_LINKER_FLAGS_INIT=-fuse-ld=lld"' />
+  </ItemGroup>
+
+  <ItemGroup>
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm64' and '$(BuildOS)' == 'Windows_NT'" Include="-DLLVM_DEFAULT_TARGET_TRIPLE=aarch64-windows-msvc" />
     <_LLVMBuildArgs Condition="'$(ClangTarget)' != '' and '$(BuildOS)' == 'Linux'" Include="-DLLVM_DEFAULT_TARGET_TRIPLE=$(ClangTarget)" />
     <_LLVMBuildArgs Condition="'$(ClangTarget)' != '' and '$(BuildOS)' == 'Linux'" Include="-DCMAKE_OBJCOPY=$(ClangBinDir)/llvm-objcopy" />
@@ -62,8 +75,6 @@
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'x64' and '$(BuildOS)' == 'OSX'" Include="-DCMAKE_OSX_ARCHITECTURES=x86_64"/>
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm64' and '$(BuildOS)' == 'OSX'" Include="-DLLVM_DEFAULT_TARGET_TRIPLE=arm64-apple-darwin" />
     <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm64' and '$(BuildOS)' == 'OSX'" Include="-DCMAKE_OSX_ARCHITECTURES=arm64"/>
-    <!-- Enable CMake cross-compilation support by setting CMAKE_SYSTEM_NAME manually, see https://github.com/llvm/llvm-project/issues/52819 -->
-    <_LLVMBuildArgs Condition="'$(TargetArchitecture)' == 'arm64' and '$(BuildOS)' == 'Windows_NT'" Include="-DCMAKE_SYSTEM_NAME=Windows" />
     <_LLVMBuildArgs Condition="'$(BuildOS)' == 'Linux'" Include="-DCMAKE_SYSTEM_NAME=Linux" />
     <_LLVMBuildArgs Condition="'$(BuildOS)' == 'OSX'" Include="-DCMAKE_SYSTEM_NAME=Darwin" />
     <_LLVMBuildArgs Condition="'$(BuildOS)' == 'FreeBSD'" Include="-DCMAKE_SYSTEM_NAME=FreeBSD" />
@@ -118,9 +129,9 @@
   </PropertyGroup>
 
   <ItemGroup Condition="'$(BuildOS)' != 'Windows_NT'">
-    <_LLVMBuildArgs Include='-DCMAKE_C_FLAGS="-I../llvm/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags)"' />
-    <_LLVMBuildArgs Include='-DCMAKE_CXX_FLAGS="-I../llvm/include $(_LibCxxCFlags) -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags) "' />
-    <_LLVMBuildArgs Include='-DCMAKE_ASM_FLAGS="-I../llvm/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags) "' />
+    <_LLVMBuildArgs Include='-DCMAKE_C_FLAGS="-I../llvm/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags)"' />
+    <_LLVMBuildArgs Include='-DCMAKE_CXX_FLAGS="-I../llvm/include $(_LibCxxCFlags) -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags) "' />
+    <_LLVMBuildArgs Include='-DCMAKE_ASM_FLAGS="-I../llvm/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags) "' />
     <_LLVMBuildArgs Include='-DCMAKE_EXE_LINKER_FLAGS_INIT="$(_SharedLinkerFlags)"' />
     <_LLVMBuildArgs Include='-DCMAKE_SHARED_LINKER_FLAGS_INIT="$(_SharedLinkerFlags)"' />
     <_LLVMBuildArgs Include='-DCMAKE_MODULE_LINKER_FLAGS_INIT="$(_SharedLinkerFlags)"' />
@@ -139,9 +150,9 @@
       <_LibCxxBuildArgs Include="-DCMAKE_C_COMPILER=clang" />
       <_LibCxxBuildArgs Include="-DCMAKE_CXX_COMPILER=clang++" />
       <_LibCxxBuildArgs Include="-DCMAKE_ASM_COMPILER=clang" />
-      <_LibCxxBuildArgs Include='-DCMAKE_C_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags)"' />
-      <_LibCxxBuildArgs Include='-DCMAKE_CXX_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags)"' />
-      <_LibCxxBuildArgs Include='-DCMAKE_ASM_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CrossCFlags)"' />
+      <_LibCxxBuildArgs Include='-DCMAKE_C_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags)"' />
+      <_LibCxxBuildArgs Include='-DCMAKE_CXX_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags)"' />
+      <_LibCxxBuildArgs Include='-DCMAKE_ASM_FLAGS="-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DNDEBUG -D__NO_CTYPE_INLINE -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS $(_CFlags)"' />
       <_LibCxxBuildArgs Include='-DCMAKE_EXE_LINKER_FLAGS_INIT="-Wl,--build-id -fuse-ld=lld"' />
       <_LibCxxBuildArgs Include='-DCMAKE_SHARED_LINKER_FLAGS_INIT="-Wl,--build-id -fuse-ld=lld"' />
       <_LibCxxBuildArgs Include='-DCMAKE_MODULE_LINKER_FLAGS_INIT="-Wl,--build-id -fuse-ld=lld"' />
@@ -151,18 +162,22 @@
       <_LibCxxBuildArgs Include="-DLLVM_DEFAULT_TARGET_TRIPLE=$(ClangTarget)" />
       <_LibCxxBuildArgs Condition="$(ClangTarget.ToLowerInvariant().Contains('musl'))" Include="-DLIBCXX_HAS_MUSL_LIBC=ON" />
     </ItemGroup>
+    <PropertyGroup>
+      <_LibCxxBuildCommand Condition="'$(CMakeGenerator)' == 'Unix Makefiles'">$(_SetupEnvironment) make -j$([System.Environment]::ProcessorCount)</_LibCxxBuildCommand>
+      <_LibCxxBuildCommand Condition="'$(CMakeGenerator)' == 'Ninja'">$(_SetupEnvironment) ninja</_LibCxxBuildCommand>
+    </PropertyGroup>
     <Message Text="** Configuring bootstrap libc++ '$(_LibCxxConfigureCommand)'" Importance="High" />
     <Exec WorkingDirectory="$(_LibCxxBuildDir)"
           Command="$(_LibCxxConfigureCommand)"
           Condition="!Exists('$(_LibCxxBuildDir)/CMakeCache.txt')"
           IgnoreStandardErrorWarningFormat="true" />
-    <Message Text="** Building bootstrap libc++ '$(_BuildCommand)'" Importance="High" />
+    <Message Text="** Building bootstrap libc++ '$(_LibCxxBuildCommand)'" Importance="High" />
     <Exec WorkingDirectory="$(_LibCxxBuildDir)"
-          Command="$(_BuildCommand)"
+          Command="$(_LibCxxBuildCommand)"
           IgnoreStandardErrorWarningFormat="true" />
-    <Message Text="** Installing bootstrap libc++ '$(_CMakeInstallCommand)'" Importance="High" />
+    <Message Text="** Installing bootstrap libc++ '$(_LibCxxBuildCommand) install'" Importance="High" />
     <Exec WorkingDirectory="$(_LibCxxBuildDir)"
-          Command="$(_BuildCommand) install"
+          Command="$(_LibCxxBuildCommand) install"
           IgnoreStandardErrorWarningFormat="true" />
     <ItemGroup>
       <_LibCxxUnstripped Include="libc++.so.1.0" />
diff --git a/nuget/packages.builds b/nuget/packages.builds
index 8bd384023bd7b..e08c87f3a6e43 100644
--- a/nuget/packages.builds
+++ b/nuget/packages.builds
@@ -1,16 +1,19 @@
 <Project Sdk="Microsoft.Build.Traversal">
-  <ItemGroup>
+  <ItemGroup Condition="'$(BuildObjWriterOnly)' != 'true'">
     <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Sdk\Microsoft.NETCore.Runtime.Mono.LLVM.Sdk.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Tools\Microsoft.NETCore.Runtime.Mono.LLVM.Tools.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Wasm.A.Transport\Microsoft.NETCore.Runtime.Mono.LLVM.Wasm.A.Transport.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Wasm.B.Transport\Microsoft.NETCore.Runtime.Mono.LLVM.Wasm.B.Transport.builds" />
-    <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.ObjWriter\Microsoft.NETCore.Runtime.ObjWriter.builds" />
-    <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.JIT.Tools\Microsoft.NETCore.Runtime.JIT.Tools.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.Wasm.LLVM.Transport\Microsoft.NETCore.Runtime.Wasm.LLVM.Transport.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Debug'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Sdk\Microsoft.NETCore.Runtime.Mono.LLVM.Sdk.Debug.builds" />
     <ProjectReference Condition="'$(Configuration)' == 'Debug'" Include="Microsoft.NETCore.Runtime.Mono.LLVM.Tools\Microsoft.NETCore.Runtime.Mono.LLVM.Tools.Debug.builds" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.ObjWriter\Microsoft.NETCore.Runtime.ObjWriter.builds" />
+    <ProjectReference Condition="'$(Configuration)' == 'Release'" Include="Microsoft.NETCore.Runtime.JIT.Tools\Microsoft.NETCore.Runtime.JIT.Tools.builds" />
+  </ItemGroup>
+
   <!-- Generate a version.txt file we include in our packages
        The InitializeSourceControlInformationFromSourceControlManager is part of Microsoft.Build.Tasks.Git
        and is responsible for setting SourceRevisionId -->