On Wed, 27 May 2015, AKASHI Takahiro wrote:
quoted
quoted
Livepatch was merged in v4.0, and allows replacying a function dynamically
based on ftrace framework, but it also requires -mfentry option of gcc.
Currently arm64 gcc doesn't support it, but by adding a helper function to
ftrace, we will be able to support livepatch on arch's which don't support
this option.
I submit this patchset as RFC since I'm not quite sure that I'm doing
in the right way, or we should definitely support -fentry instead.
I don't have arm64 cross-compiler handy, could you please copy/paste how
does function prologue, generated by gcc -pg on arm64 look like?
As other people said, my current patch has some drawbacks and was far
from perfect. I talked to a toolchain guy in Linaro, and he suggested
that, instead of x86 specific -mfentry option, we should add a new
generic option, -fprolog-pad=N.
It works as if "-pg -mfentry -mrecord-mcount -mnop-mcount" were
specified on a command line, but actually puts N nop instructions at the
very beginning of a function. (Please note that -mrecord-mcount and
-mnop-mcount options have been added in x86 gcc to make it easier to
implement ftrace and others.)
This is BTW implemented in gcc for various architectures already (see
-mhotpatch for s390 or -mprofile-kernel for ppc64). Would be nice to at
least try the semantics and format unified in some sense.
Thanks,
--
Jiri Kosina
SUSE Labs