Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

[Sami Tolvanen <samitolvanen@google.com>]

On Fri, Nov 20, 2020 at 3:59 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Nov 20, 2020 at 12:58:41PM -0800, Sami Tolvanen wrote:
> > On Fri, Nov 20, 2020 at 12:43 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > On Fri, Nov 20, 2020 at 01:29:35PM -0700, Nathan Chancellor wrote:
> > > > On Fri, Nov 20, 2020 at 11:47:21AM -0800, Kees Cook wrote:
> > > > > On Fri, Nov 20, 2020 at 08:23:11AM -0800, Sami Tolvanen wrote:
> > > > > > Changing the ThinLTO config to a choice and moving it after the main
> > > > > > LTO config sounds like a good idea to me. I'll see if I can change
> > > > > > this in v8. Thanks!
> > > > >
> > > > > Originally, I thought this might be a bit ugly once GCC LTO is added,
> > > > > but this could be just a choice like we're done for the stack
> > > > > initialization. Something like an "LTO" choice of NONE, CLANG_FULL,
> > > > > CLANG_THIN, and in the future GCC, etc.
> > > >
> > > > Having two separate choices might be a little bit cleaner though? One
> > > > for the compiler (LTO_CLANG versus LTO_GCC) and one for the type
> > > > (THINLTO versus FULLLTO). The type one could just have a "depends on
> > > > CC_IS_CLANG" to ensure it only showed up when needed.
> > >
> > > Right, that's how the stack init choice works. Kconfigs that aren't
> > > supported by the compiler won't be shown. I.e. after Sami's future
> > > patch, the only choice for GCC will be CONFIG_LTO_NONE. But building
> > > under Clang, it would offer CONFIG_LTO_NONE, CONFIG_LTO_CLANG_FULL,
> > > CONFIG_LTO_CLANG_THIN, or something.
> > >
> > > (and I assume  CONFIG_LTO would be def_bool y, depends on !LTO_NONE)
> >
> > I'm fine with adding ThinLTO as another option to the LTO choice, but
> > it would duplicate the dependencies and a lot of the help text. I
> > suppose we could add another config for the dependencies and have both
> > LTO options depend on that instead.
>
> How about something like this? This separates the arch support, compiler
> support, and user choice into three separate Kconfig areas, which I
> think should work.

Sure, this looks good to me, I'll use this in v8. The only minor
concern I have is that ThinLTO cannot be set as the default LTO mode,
but I assume anyone who selects LTO is also capable of deciding which
mode is better for them.

> diff --git a/Makefile b/Makefile
> index e397c4caec1b..af902718e882 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -897,7 +897,7 @@ export CC_FLAGS_SCS
>  endif
>
>  ifdef CONFIG_LTO_CLANG
> -ifdef CONFIG_THINLTO
> +ifdef CONFIG_LTO_CLANG_THIN
>  CC_FLAGS_LTO   += -flto=thin -fsplit-lto-unit
>  KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
>  else
> diff --git a/arch/Kconfig b/arch/Kconfig
> index cdd29b5fdb56..5c22e10e4c12 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -600,6 +600,14 @@ config SHADOW_CALL_STACK
>
>  config LTO
>         bool
> +       help
> +         Selected if the kernel will be built using the compiler's LTO feature.
> +
> +config LTO_CLANG
> +       bool
> +       select LTO
> +       help
> +         Selected if the kernel will be built using Clang's LTO feature.
>
>  config ARCH_SUPPORTS_LTO_CLANG
>         bool
> @@ -609,28 +617,25 @@ config ARCH_SUPPORTS_LTO_CLANG
>           - compiling inline assembly with Clang's integrated assembler,
>           - and linking with LLD.
>
> -config ARCH_SUPPORTS_THINLTO
> +config ARCH_SUPPORTS_LTO_CLANG_THIN
>         bool
>         help
> -         An architecture should select this option if it supports Clang's
> -         ThinLTO.
> +         An architecture should select this option if it can supports Clang's
> +         ThinLTO mode.
>
> -config THINLTO
> -       bool "Clang ThinLTO"
> -       depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
> -       default y
> +config HAS_LTO_CLANG
> +       def_bool y
> +       # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510
> +       depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> +       depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
> +       depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
> +       depends on ARCH_SUPPORTS_LTO_CLANG
> +       depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT
> +       depends on !KASAN
> +       depends on !GCOV_KERNEL
>         help
> -         This option enables Clang's ThinLTO, which allows for parallel
> -         optimization and faster incremental compiles. More information
> -         can be found from Clang's documentation:
> -
> -           https://clang.llvm.org/docs/ThinLTO.html
> -
> -         If you say N here, the compiler will use full LTO, which may
> -         produce faster code, but building the kernel will be significantly
> -         slower as the linker won't efficiently utilize multiple threads.
> -
> -         If unsure, say Y.
> +         The compiler and Kconfig options support building with Clang's
> +         LTO.
>
>  choice
>         prompt "Link Time Optimization (LTO)"
> @@ -644,20 +649,14 @@ choice
>
>  config LTO_NONE
>         bool "None"
> +       help
> +         Build the kernel normally, without Link Time Optimization (LTO).
>
> -config LTO_CLANG
> -       bool "Clang's Link Time Optimization (EXPERIMENTAL)"
> -       # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510
> -       depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> -       depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
> -       depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
> -       depends on ARCH_SUPPORTS_LTO_CLANG
> -       depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT
> -       depends on !KASAN
> -       depends on !GCOV_KERNEL
> -       select LTO
> +config LTO_CLANG_FULL
> +       bool "Clang Full LTO (EXPERIMENTAL)"
> +       select LTO_CLANG
>         help
> -          This option enables Clang's Link Time Optimization (LTO), which
> +          This option enables Clang's full Link Time Optimization (LTO), which
>            allows the compiler to optimize the kernel globally. If you enable
>            this option, the compiler generates LLVM bitcode instead of ELF
>            object files, and the actual compilation from bitcode happens at
> @@ -667,9 +666,22 @@ config LTO_CLANG
>
>             https://llvm.org/docs/LinkTimeOptimization.html
>
> -         To select this option, you also need to use LLVM tools to handle
> -         the bitcode by passing LLVM=1 to make.
> +         During link time, this option can use a large amount of RAM, and
> +         may take much longer than the ThinLTO option.
>
> +config LTO_CLANG_THIN
> +       bool "Clang ThinLTO (EXPERIMENTAL)"
> +       depends on ARCH_SUPPORTS_LTO_CLANG_THIN
> +       select LTO_CLANG
> +       help
> +         This option enables Clang's ThinLTO, which allows for parallel
> +         optimization and faster incremental compiles compared to the
> +         CONFIG_LTO_CLANG_FULL option. More information can be found
> +         from Clang's documentation:
> +
> +           https://clang.llvm.org/docs/ThinLTO.html
> +
> +         If unsure, say Y.
>  endchoice

The two LTO_CLANG_* options need to depend on HAS_LTO_CLANG, of course.

Sami