Re: [PATCH] powerpc: Add gpr1 and fpu save/restore functions

Timothy Pearson [off-list ref] writes:

----- Original Message -----

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From: "Segher Boessenkool" <redacted>
To: "Timothy Pearson" <tpearson@raptorengineering.com>
Cc: "linuxppc-dev" <redacted>
Sent: Monday, February 12, 2024 12:23:22 PM
Subject: Re: [PATCH] powerpc: Add gpr1 and fpu save/restore functions

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On Mon, Feb 12, 2024 at 12:07:03PM -0600, Timothy Pearson wrote:

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I have done it for *all* architectures some ten years ago.  Never found
any problem.

That makes sense, what I mean by invasive is that we'd need buy-in from the
other
maintainers across all of the affected architectures.  Is that likely to occur?

I don't know.  Here is my PowerPC-specific patch, it's a bit older, it
might not apply cleanly anymore, the changes needed should be obvious
though:


=== 8< ===
commit f16dfa5257eb14549ce22243fb2b465615085134
Author: Segher Boessenkool [off-list ref]
Date:   Sat May 3 03:48:06 2008 +0200

   powerpc: Link vmlinux against libgcc.a

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index b7212b619c52..0a2fac6ffc1c 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile

@@ -158,6 +158,9 @@ core-y                              += arch/powerpc/kernel/

core-$(CONFIG_XMON)            += arch/powerpc/xmon/
core-$(CONFIG_KVM)             += arch/powerpc/kvm/

+LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
+libs-y += $(LIBGCC)
+
drivers-$(CONFIG_OPROFILE)     += arch/powerpc/oprofile/

# Default to zImage, override when needed
=== 8< ===

OK.  PowerPC maintainers, how would you prefer to handle this?

I'll take the patch to add the functions for now. We can look into
linking against libgcc as a future cleanup.

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There are better options than -Os, fwiw.  Some --param's give smaller
*and* faster kernels.  What exactly is best is heavily arch-dependent
though (as well as dependent on the application code, the kernel code in
this case) :-(

I've been through this a few times, and -Os is the only option that makes
things (just barely) fit unfortunately.

-O2 with appropriate inlining tuning beats -Os every day of the week,
in my experience.

On 6.6 it's 24MiB vs 40MiB, O2 vs. Os. :(

What compiler/config etc. are you using for that?

I see almost no difference, though the defconfig (which uses -O2) is
actually smaller:

$ ls -l vmlinux.Os vmlinux.defconfig
-rwxr-xr-x. 1 michael michael 49936640 Feb 13 16:11 vmlinux.defconfig*
-rwxr-xr-x. 1 michael michael 50108392 Feb 13 16:14 vmlinux.Os*

cheers