Re: arm64:, Re: [RFC] Kernel livepatching support in GCC

[AKASHI Takahiro <takahiro dot akashi at linaro dot org>]

On 10/22/2015 07:26 PM, Szabolcs Nagy wrote:
> On 22/10/15 11:14, AKASHI Takahiro wrote:
> > On 10/22/2015 06:07 PM, libin wrote:
> > > å 2015/5/28 16:39, Maxim Kuvyrkov åé:
> > > > Our proposal is that instead of adding -mfentry/-mnop-count/-mrecord-mcount options to other architectures,
> > > > we should
> > > > implement a target-independent option -fprolog-pad=N, which will generate a pad of N nops at the beginning
> > > > of each
> > > > function and add a section entry describing the pad similar to -mrecord-mcount [1].
> > > >
> > > > Since adding NOPs is much less architecture-specific then outputting call instruction sequences, this option
> > > > can be
> > > > handled in a target-independent way at least for some/most architectures.
> > > >
> > > > Comments?
> > > >
> > > > As I found out today, the team from Huawei has implemented [2], which follows x86 example of -mfentry option
> > > > generating a hard-coded call sequence.  I hope that this proposal can be easily incorporated into their work
> > > > since
> > > > most of the livepatching changes are in the kernel.
> > >
> > > Thanks very much for your effort for this, and the arch-independed implementation
> > > is very good to me, but only have one question that how to enture the atomic
> > > replacement of multi instructions in kernel side?
> >
> > I have one idea, but we'd better discuss this topic in, at least including, linux-arm-kernel.
> >
> > > And before this arch-independed option, can we consider the arch-depended -mfentry
> > > implemention for arm64 like arch x86 firstly? I will post it soon.
> > >
> > > livepatch for arm64 based on this arm64 -mfentry feature on github:
> > > https://github.com/libin2015/livepatch-for-arm64.git  master
> >
> >
> > I also have my own version of livepatch support for arm64 using yet-coming "-fprolog-add=N" option :)
> > As we discussed before, the main difference will be how we should preserve LR register when invoking
> > a ftrace hook (ftrace_regs_caller).
> > But again, this is a topic to discuss mainly in linux-arm-kernel.
> > (I have no intention of excluding gcc ml from the discussions.)
>
> is -fprolog-add=N enough from gcc?

Yes, as far as I correctly understand this option.

> i assume it solves the live patching, but i thought -mfentry
> might be still necessary when live patching is not used.

No.
- Livepatch depends on ftrace's DYNAMIC_FTRACE_WITH_REGS feature
- DYNAMIC_FTRACE_WITH_REGS can be implemented either with -fprolog-add=N or -mfentry
- x86 is the only architecture that supports -mfentry AFAIK
- and it is used in the kernel solely to implement this ftrace feature AFAIK
- So once a generic option, fprolog-add=N, is supported, we have no reason to add arch-specific -mfentry.

> or is the kernel fine with the current mcount abi for that?
> (note that changes the code generation in leaf functions

Can you please elaborate your comments in more details?
I didn't get your point here.

Thanks,
-Takahiro AKASHI

> and currently the kernel relies on frame pointers etc.)