Re: [RFC 0/4] arm64: add livepatch support

[Jiri Kosina]

On Wed, 27 May 2015, AKASHI Takahiro wrote:

> > > 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
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