Re: [PATCH 00/46] gcc-LTO support for the kernel

[Peter Zijlstra]

On Thu, Nov 17, 2022 at 08:50:59AM +0000, Richard Biener wrote:
> On Thu, 17 Nov 2022, Peter Zijlstra wrote:
> 
> > On Mon, Nov 14, 2022 at 08:40:50PM +0100, Ard Biesheuvel wrote:
> > > On Mon, 14 Nov 2022 at 12:44, Jiri Slaby (SUSE) <jirislaby@xxxxxxxxxx> wrote:
> > > >
> > > > Hi,
> > > >
> > > > this is the first call for comments (and kbuild complaints) for this
> > > > support of gcc (full) LTO in the kernel. Most of the patches come from
> > > > Andi. Me and Martin rebased them to new kernels and fixed the to-use
> > > > known issues. Also I updated most of the commit logs and reordered the
> > > > patches to groups of patches with similar intent.
> > > >
> > > > The very first patch comes from Alexander and is pending on some x86
> > > > queue already (I believe). I am attaching it only for completeness.
> > > > Without that, the kernel does not boot (LTO reorders a lot).
> > > >
> > > > In our measurements, the performance differences are negligible.
> > > >
> > > > The kernel is bigger with gcc LTO due to more inlining.
> > > 
> > > OK, so if I understand this correctly:
> > > - the performance is the same
> > > - the resulting image is bigger
> > > - we need a whole lot of ugly hacks to placate the linker.
> > > 
> > > Pardon my cynicism, but this cover letter does not mention any
> > > advantages of LTO, so what is the point of all of this?
> > 
> > Seconded; I really hate all the ugly required for the GCC-LTO
> > 'solution'. There not actually being any benefit just makes it a very
> > simple decision to drop all these patches on the floor.
> 
> I'd say that instead a prerequesite for the series would be to actually
> enforce hidden visibility for everything not part of the kernel module
> API so the compiler can throw away unused functions.  Currently it has
> to keep everything because with a shared object there might be external
> references to everything exported from individual TUs.

I'm not sure what you're on about; only symbols annotated with
EXPORT_SYMBOL*() are accessible from modules (aka DSOs) and those will
have their address taken. You can feely eliminate any unused symbol.

> There was a size benefit mentioned for module-less monolithic kernels
> as likely used in embedded setups, not sure if that's enough motivation
> to properly annotate symbols with visibility - and as far as I understand
> all these 'required' are actually such fixes.

I'm not seeing how littering __visible is useful or desired, doubly so
for that static hack, that's just a crude work around for GCC LTO being
inferior for not being able to read inline asm.