Re: [PATCH 0/4] powerpc/64: ELFv2 conversion

From: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 0/4] powerpc/64: ELFv2 conversion
Date: Fri, 05 May 2023 20:38:06 +0530	[thread overview]
Message-ID: <1683298879.x90su0d4vv.naveen@linux.ibm.com> (raw)
In-Reply-To: <87wn1n55qc.fsf@mail.lhotse>

Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>> Le 05/05/2023 à 09:18, Nicholas Piggin a écrit :
>>> This series has the steps to remove ELFv1 from the kernel build.
>>> Patch 1 is a build fix, 2 defaults everything to ELFv2, 3 is
>>> really a separate issue that concerns userspace. 4 removes v1
>>> support.

Super glad to see this!

>>
>> I see CONFIG_MPROFILE_KERNEL is restricted to LITTLE_ENDIAN. Is that 
>> correct ? Don't we have mprofile_kernel as well on BIG ENDIAN once 
>> switched to ABI v2 ? If so, can we drop the -pg based profiling 
>> completely, or is CLANG still not supporting mprofile_kernel ?
> 
> Clang does not support -mprofile-kernel.
> 
> AIUI they would like us to switch to -fpatchable-function-entry which
> could then be supported on GCC & clang.
> 
> That would be a nice cleanup but also a bunch of work that no one has
> found time to do.
> 
> https://github.com/linuxppc/issues/issues/447
> https://github.com/llvm/llvm-project/issues/57031

GCC's support for -fpatchable-function-entry needed fixes for ABIv2, 
which have landed in the recent v13.1 release. I am looking into adding 
support for that.

> 
> 
> So for now we need to keep the -pg based version.

Indeed, but removing elf v1 support does help cleaning up and 
simplifying some of the tracing code. Much of the -pg code is already 
separate, so it should be easy to drop that once clang picks up support 
for -fpatchable-function-entry.

- Naveen