linux-kernel - Re: [bisected] clang 15 built kernel fails to boot, stuck at "Loading Linux 6.1.1 ...", gcc 12 built kernel with same config boots fine

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Date:   Fri, 20 Jan 2023 12:16:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sandipan Das <sandipan.das@....com>
Cc:     "Erhard F." <erhard_f@...lbox.org>, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>
Subject: Re: [bisected] clang 15 built kernel fails to boot, stuck at
 "Loading Linux 6.1.1 ...", gcc 12 built kernel with same config boots fine

On Fri, Jan 20, 2023 at 11:20:53AM +0530, Sandipan Das wrote:

> With the combination of defconfig+kvm_guest.config+localyesconfig, the only
> thing that made a difference was the compiler optimization choice. The kernel
> boots up with CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y but not with
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y. Both Clang 15 and 16 kernel builds are affected.

*groan*..

0000000000000350 <amd_pmu_add_event>:
350:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1) 351: R_X86_64_NONE      __fentry__-0x4
355:       48 83 bf 20 01 00 00 00         cmpq   $0x0,0x120(%rdi)
35d:       0f 85 00 00 00 00       jne    363 <amd_pmu_add_event+0x13>     35f: R_X86_64_PLT32     __SCT__amd_pmu_branch_add-0x4
363:       e9 00 00 00 00          jmp    368 <amd_pmu_add_event+0x18>     364: R_X86_64_PLT32     __x86_return_thunk-0x4

and static_call() can't deal with Jcc, I wonder why we've not seen that
before -- this isn't totally idiotic code-gen and esp. clang is known to
do this (see the retpoline thing).

Let me see if I can do something about that.