[v8,00/10] x86: macrofying inline asm for better compilation

Message

Nadav Amit Sept. 18, 2018, 9:28 p.m. UTC

This patch-set deals with an interesting yet stupid problem: kernel code
that does not get inlined despite its simplicity. There are several
causes for this behavior: "cold" attribute on __init, different function
optimization levels; conditional constant computations based on
__builtin_constant_p(); and finally large inline assembly blocks.

This patch-set deals with the inline assembly problem. I separated these
patches from the others (that were sent in the RFC) for easier
inclusion. I also separated the removal of unnecessary new-lines which
would be sent separately.

The problem with inline assembly is that inline assembly is often used
by the kernel for things that are other than code - for example,
assembly directives and data. GCC however is oblivious to the content of
the blocks and assumes their cost in space and time is proportional to
the number of the perceived assembly "instruction", according to the
number of newlines and semicolons. Alternatives, paravirt and other
mechanisms are affected, causing code not to be inlined, and degrading
compilation quality in general.

The solution that this patch-set carries for this problem is to create
an assembly macro, and then call it from the inline assembly block.  As
a result, the compiler sees a single "instruction" and assigns the more
appropriate cost to the code.

To avoid uglification of the code, as many noted, the macros are first
precompiled into an assembly file, which is later assembled together
with the C files. This also enables to avoid duplicate implementation
that was set before for the asm and C code. This can be seen in the
exception table changes.

Overall this patch-set slightly increases the kernel size (my build was
done using my Ubuntu 18.04 config + localyesconfig for the record):

   text	   data	    bss	    dec	    hex	filename
18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
18163608 10227348 2957312 31348268 1de562c ./vmlinux after (+0.1%)

The number of static functions in the image is reduced by 379, but
actually inlining is even better, which does not always shows in these
numbers: a function may be inlined causing the calling function not to
be inlined.

I ran some limited number of benchmarks, and in general the performance
impact is not very notable. You can still see >10 cycles shaved off some
syscalls that manipulate page-tables (e.g., mprotect()), in which
paravirt caused many functions not to be inlined. In addition this
patch-set can prevent issues such as [1], and improves code readability
and maintainability.

[1] https://patchwork.kernel.org/patch/10450037/

v7->v8:	* Add acks (Masahiro, Max)
	* Rebase on 4.19 (Ingo)

v6->v7: * Fix context switch tracking (Ingo)
	* Fix xtensa build error (Ingo)
	* Rebase on 4.18-rc8

v5->v6:	* Removing more code from jump-labels (PeterZ)
	* Fix build issue on i386 (0-day, PeterZ)

v4->v5:	* Makefile fixes (Masahiro, Sam)

v3->v4: * Changed naming of macros in 2 patches (PeterZ)
	* Minor cleanup of the paravirt patch

v2->v3: * Several build issues resolved (0-day)
	* Wrong comments fix (Josh)
	* Change asm vs C order in refcount (Kees)

v1->v2:	* Compiling the macros into a separate .s file, improving
	  readability (Linus)
	* Improving assembly formatting, applying most of the comments
	  according to my judgment (Jan)
	* Adding exception-table, cpufeature and jump-labels
	* Removing new-line cleanup; to be submitted separately

Cc: Masahiro Yamada <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Cc: Alok Kataria <[e-mail]>
Cc: Christopher Li <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Kate Stewart <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: [e-mail]
Cc: Peter Zijlstra <[e-mail]>
Cc: Philippe Ombredanne <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: Linus Torvalds <[e-mail]>
Cc: [e-mail]
Cc: Chris Zankel <[e-mail]>
Cc: Max Filippov <[e-mail]>
Cc: [e-mail]

Nadav Amit (10):
  xtensa: defining LINKER_SCRIPT for the linker script
  Makefile: Prepare for using macros for inline asm
  x86: objtool: use asm macro for better compiler decisions
  x86: refcount: prevent gcc distortions
  x86: alternatives: macrofy locks for better inlining
  x86: bug: prevent gcc distortions
  x86: prevent inline distortion by paravirt ops
  x86: extable: use macros instead of inline assembly
  x86: cpufeature: use macros instead of inline assembly
  x86: jump-labels: use macros instead of inline assembly

 Makefile                               |  9 ++-
 arch/x86/Makefile                      | 11 ++-
 arch/x86/entry/calling.h               |  2 +-
 arch/x86/include/asm/alternative-asm.h | 20 ++++--
 arch/x86/include/asm/alternative.h     | 11 +--
 arch/x86/include/asm/asm.h             | 61 +++++++---------
 arch/x86/include/asm/bug.h             | 98 +++++++++++++++-----------
 arch/x86/include/asm/cpufeature.h      | 82 ++++++++++++---------
 arch/x86/include/asm/jump_label.h      | 77 ++++++++------------
 arch/x86/include/asm/paravirt_types.h  | 56 +++++++--------
 arch/x86/include/asm/refcount.h        | 74 +++++++++++--------
 arch/x86/kernel/macros.S               | 16 +++++
 arch/xtensa/kernel/Makefile            |  4 +-
 include/asm-generic/bug.h              |  8 +--
 include/linux/compiler.h               | 56 +++++++++++----
 scripts/Kbuild.include                 |  4 +-
 scripts/mod/Makefile                   |  2 +
 17 files changed, 333 insertions(+), 258 deletions(-)
 create mode 100644 arch/x86/kernel/macros.S

Comments

Kees Cook Sept. 21, 2018, 6:26 p.m. UTC | #1

On Tue, Sep 18, 2018 at 2:28 PM, Nadav Amit <[e-mail]> wrote:
> This patch-set deals with an interesting yet stupid problem: kernel code
> that does not get inlined despite its simplicity. There are several
> causes for this behavior: "cold" attribute on __init, different function
> optimization levels; conditional constant computations based on
> __builtin_constant_p(); and finally large inline assembly blocks.
>
> This patch-set deals with the inline assembly problem. I separated these
> patches from the others (that were sent in the RFC) for easier
> inclusion. I also separated the removal of unnecessary new-lines which
> would be sent separately.
>
> The problem with inline assembly is that inline assembly is often used
> by the kernel for things that are other than code - for example,
> assembly directives and data. GCC however is oblivious to the content of
> the blocks and assumes their cost in space and time is proportional to
> the number of the perceived assembly "instruction", according to the
> number of newlines and semicolons. Alternatives, paravirt and other
> mechanisms are affected, causing code not to be inlined, and degrading
> compilation quality in general.
>
> The solution that this patch-set carries for this problem is to create
> an assembly macro, and then call it from the inline assembly block.  As
> a result, the compiler sees a single "instruction" and assigns the more
> appropriate cost to the code.
>
> To avoid uglification of the code, as many noted, the macros are first
> precompiled into an assembly file, which is later assembled together
> with the C files. This also enables to avoid duplicate implementation
> that was set before for the asm and C code. This can be seen in the
> exception table changes.
>
> Overall this patch-set slightly increases the kernel size (my build was
> done using my Ubuntu 18.04 config + localyesconfig for the record):
>
>    text    data     bss     dec     hex filename
> 18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
> 18163608 10227348 2957312 31348268 1de562c ./vmlinux after (+0.1%)
>
> The number of static functions in the image is reduced by 379, but
> actually inlining is even better, which does not always shows in these
> numbers: a function may be inlined causing the calling function not to
> be inlined.
>
> I ran some limited number of benchmarks, and in general the performance
> impact is not very notable. You can still see >10 cycles shaved off some
> syscalls that manipulate page-tables (e.g., mprotect()), in which
> paravirt caused many functions not to be inlined. In addition this
> patch-set can prevent issues such as [1], and improves code readability
> and maintainability.
>
> [1] https://patchwork.kernel.org/patch/10450037/
>
> v7->v8: * Add acks (Masahiro, Max)
>         * Rebase on 4.19 (Ingo)

I've tested the series for booting and with the refcount lkdtm tests.
Looks good, thanks!

Tested-by: Kees Cook <[e-mail]>

-Kees