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

On Tue, Sep 18, 2018 at 2:28 PM, Nadav Amit [off-list ref] 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 <redacted>

-Kees

-- 
Kees Cook
Pixel Security