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References: <cover.1657301423.git.christophe.leroy@csgroup.eu>
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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Sat, 9 Jul 2022 08:52:33 +0200
Message-ID: <CAMj1kXFqs=YAbTDJOgzpse9ZkggSxPNNJJphEA=J94FQzF55qg@mail.gmail.com>
Subject: Re: [PATCH v2 0/7] Implement inline static calls on PPC32 - v2
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>, sv@linux.ibm.com,
        agust@denx.de, Josh Poimboeuf <jpoimboe@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jason Baron <jbaron@akamai.com>,
        "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@lists.ozlabs.org>, X86 ML <x86@kernel.org>,
        Chen Zhongjin <chenzhongjin@huawei.com>
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Hello Christophe,

On Fri, 8 Jul 2022 at 19:32, Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
> This series applies on top of the series v3 "objtool: Enable and
> implement --mcount option on powerpc" [1] rebased on powerpc-next branch
>
> A few modifications are done to core parts to enable powerpc
> implementation:
> - R_X86_64_PC32 is abstracted to R_REL32 so that it can then be
> redefined as R_PPC_REL32.
> - A call to static_call_init() is added to start_kernel() to avoid
> every architecture to have to call it
> - Trampoline address is provided to arch_static_call_transform() even
> when setting a site to fallback on a call to the trampoline when the
> target is too far.
>
> [1] https://lore.kernel.org/lkml/70b6d08d-aced-7f4e-b958-a3c7ae1a9319@csgroup.eu/T/#rb3a073c54aba563a135fba891e0c34c46e47beef
>
> Christophe Leroy (7):
>   powerpc: Add missing asm/asm.h for objtool
>   objtool/powerpc: Activate objtool on PPC32
>   objtool: Add architecture specific R_REL32 macro
>   objtool/powerpc: Add necessary support for inline static calls
>   init: Call static_call_init() from start_kernel()
>   static_call_inline: Provide trampoline address when updating sites
>   powerpc/static_call: Implement inline static calls
>

Could you quantify the performance gains of moving from out-of-line,
patched tail-call branch instructions to full-fledged inline static
calls? On x86, the retpoline problem makes this glaringly obvious, but
on other architectures, the complexity of supporting this model may
outweigh the performance advantages.