Re: [RFC PATCH] configure: Enable -fno-omit-frame-pointer for backtrace module

[Daniel Kiper]

On Mon, Aug 14, 2023 at 12:47:33AM -0500, Glenn Washburn wrote:
> On Sat, 12 Aug 2023 01:57:42 +0200
> "Vladimir 'phcoder' Serbinenko" <phcoder@gmail.com> wrote:
>
> > Le sam. 12 août 2023, 00:22, Glenn Washburn <development@efficientek.com> a
> > écrit :
> >
> > > The backtrace module is written assuming that the frame pointer is in 
> > > %ebp.
> > > By default, -Os optimization level is used, which enables the gcc option
> > > -fomit-frame-pointer. This break the backtrace functionality. Enabling
> > > this may cause an unnoticeable performance cost and virtually no size
> > > increase.
> > >
> > Do you have numbers on core.img size on i386-pc?
>
> No, I don't because I'm not really using i386-pc. However, do these numbers 
> work
> for you instead? Or are you wanting to see if the core.img gets too large?
>
> ==== With frame pointer
> $ du -bc grub-i386-pc/lib/grub/i386-pc/*.mod | tail -n 1
> 1932852 total
> $ du -bc grub-i386-pc/lib/grub/i386-pc/kernel.img
> 34672   grub-i386-pc/lib/grub/i386-pc/kernel.img
> 43152   total
>
> ==== No frame pointer
> $ du -bc grub-i386-pc/lib/grub/i386-pc/*.mod | tail -n 1
> 1923236 total
> $ du -bc grub-i386-pc/lib/grub/i386-pc/kernel.img
> 34512   grub-i386-pc/lib/grub/i386-pc/kernel.img

I would not bother much here because we do not support small MBR gaps for
quite long time. So, Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>...

Though...

> Glenn
>
> > >
> > > Signed-off-by: Glenn Washburn <development@efficientek.com>
> > > ---
> > > The backtrace command on x86_64 and probably i386 is broken due to the
> > > above rationale. I've not verified, but presumably the backtrace that used
> > > to be printed for an unhandled CPU exception is also broken. Do any 
> > > distros
> > > handle this?
> > >
> > > Considering that (to my knowledge) no one has complained about this in the
> > > over 13 years that -Os has been used, has this code actually been useful?
> > > Is it worth disabling -fomit-frame-pointer? Though, I don't see much
> > > downside
> > > right now in disabling it. Alternatively, we could disable/remove the
> > > backtrace code. I think it would be nice to keep it and have it working.
> > >
> > > Nowadays, presumably QEMU makes the GDB stub rarely used as I imagine most
> > > are developing in a virual machine. Also, the GDB stub does not work in
> > > UEFI
> > > so if anyone is using it on real hardware, they are doing so on pretty old
> > > machines. The lack of a GDB stub does not seem to be a pain point because
> > > no one has got it working on UEFI.
> > >
> > > This patch gets the backtrace command working on x86_64-efi in QEMU for 
> > > me.
> > > However, it hangs when run on my laptop. Not sure what's going on there.

... I would move everything up until here to the commit message. If you are
OK with it I can do it for you.

> > > Also, I've noticed that recents builds have caused a big slow down in the
> > > already slow LUKS unlock code on real hardware, but not noticeable in 
> > > QEMU.
> > > It seems to occur with and without this patch. I mention this if others
> > > would be interested in testing this.

Daniel