102 lines
4.6 KiB
Markdown
102 lines
4.6 KiB
Markdown
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As of Android S, this deprecation is fully complete (except for ARCH=arm on
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android-4.19-stable) and step 9 which is being cleaned up for Android T.
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The relevant kernel commits are:
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v5.7-rc1:
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commit a0d1c951ef08 ("kbuild: support LLVM=1 to switch the default tools to Clang/LLVM")
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v5.15-rc1:
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commit f12b034afeb3 ("scripts/Makefile.clang: default to LLVM_IAS=1")
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The original version of this document is retained below:
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---
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In the effort to improve test coverage of the LLVM substitutes when building
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Linux kernels for Android distributions, as well as minimize build
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dependencies, we plan to phase kernel builds over to use the LLVM substitutes
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distributed through AOSP for Android S. This will remove the kernels from being
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dependent on binutils in Android. The NDK is currently the other largest
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dependency.
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Invoking a kernel build with all of the substitutes is tedious; see
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https://www.kernel.org/doc/html/latest/kbuild/llvm.html#llvm-utilities, so
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`make LLVM=1` was introduced in:
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commit a0d1c951ef08 ("kbuild: support LLVM=1 to switch the default tools to Clang/LLVM")
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which first landed in v5.7-rc1 and will need to be backported to at least 5.4.
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Support for using Clang's "integrated assembler" is a risk; we have gotten it
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working upstream, but then small changes to assembly quickly uncover missing
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support. Thus `LLVM_IAS=1` is a separate flag from `LLVM=1`.
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The plan for S is:
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1. Ensure a0d1c951ef08 is backported to all supported kernel version branches
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for S.
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2. Wire up support in Android common kernel's build/build.sh to forward
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`LLVM=1` to `make` from a supplied config. `LLVM=1` needs to be specified
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for all invocations of `make`.
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3. Update kernel configs to use `LLVM=1` and remove the individual flags like
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`CC=clang`, `NM=llvm-nm`, and such.
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4. Create a branch of
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https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/
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that only contains the GNU as (assembler) binary.
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(https://android.googlesource.com/platform/prebuilts/gas/linux-x86/)
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5. Update repo manifests for kernels to use the branch from 4, and ensure
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kernels build cleanly.
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6. Remove GNU binutils from Android kernel manifests.
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5 is potentially a lot of work, depending on whether we have a lot of fixes to
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backport or certain configs are broken that haven't been tested upstream.
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A stretch goal, assuming we get LLVM_IAS=1 in shape:
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7. Wire up support in Android common kernel's build/build.sh to forward
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`LLVM_IAS=1` to `make` from a supplied config. `LLVM_IAS=1` needs to be
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specified for all invocations of `make`. Alternatively, we can just modify
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the top level Makefile to do what LLVM_IAS=1 would do anyways, though having
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flexibility of turning it off quickly should it regress in mainline is
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probably a safer bet.
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8. Update kernel configs to use `LLVM_IAS=1`, unless we simply modified the top
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level Makefile in 6. At this point we will be using Clang's integrated
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assembler.
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9. Remove prebuilts/gas/linux-x86 (from step 4) from Android kernel manifests.
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I think we can get all 9 done for S, but 7,8,9 are a risk, and aren't critical
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to solve for S. Luckily, our comrades over at CrOS LLVM are helping whip
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Clang's integrated assembler into shape. In fact, they may end up beating us to
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the punch.
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See this public hotlist for some of the outstanding issues to be resolved.
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https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aissue+is%3Aopen+label%3A%22%5BTOOL%5D+integrated-as%22
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(Not all of those are blockers).
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The version of GNU binutils distributed by AOSP is version 2.27.0.20170315 (ToT
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GNU binutils is 2.35) plus cherry picks. The 5.9-rc1 linux kernel currently
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requires binutils 2.23 or newer.
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## Contingency Plans
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Let's say a kernel change doesn't work with Clang's integrated assembler. The
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immediate question is "does this change need to land now, or can we bear a
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revert until Clang's integrated assembler is fixed?"
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With the answer to the above question being "Yes, revert now please" then the
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contingency plan is:
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1. file a bug, or at least email android-llvm@googlegroups.com and
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ndesaulniers@google.com.
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2. revert step 9 above.
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3. revert step 8 above.
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4. Wait for toolchain feature or bug to be fixed in upstream LLVM.
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5. Wait for AOSP LLVM release containing fix.
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6. Move kernel builds to newer toolchains.
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7. Reapply step 8 above.
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8. Reapply step 9 above.
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With the answer to the above question being "No, we can wait for assembler
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support" then the contingency plan is:
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1. file a bug, or at least email android-llvm@googlegroups.com and
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ndesaulniers@google.com.
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2. Wait for toolchain feature or bug to be fixed in upstream LLVM.
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3. Wait for AOSP LLVM release containing fix.
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4. Move kernel builds to newer toolchains.
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