On Tue, Sep 18, 2018 at 2:28 PM, Nadav Amit <namit@xxxxxxxxxx> 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 <keescook@xxxxxxxxxxxx> -Kees -- Kees Cook Pixel Security
