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Date:   Fri, 20 Jan 2023 12:49:33 +0100
From:   "Erhard F." <erhard_f@mailbox.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Sandipan Das <sandipan.das@amd.com>, linux-kernel@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Joao Moreira <joao@overdrivepizza.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
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On Fri, 20 Jan 2023 12:16:26 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> 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.

I can confirm that with -O2 I get a booting kernel with netconsole output using the .config I originally posted.

However the machine does not finish booting, even with -O2. In the netconsole dmesg I get to the point "Run /sbin/init as init process" but that's it, no on-screen ouput on the laptop itself... So there seem to be 2 issues.

And sorry for my custom .config! Forgot that it was tailored for my Lenovo Thinkpad A275. Next time I'll try to replicate my issues with a more generic config (hopefully) bootable on a VM.

Regards,
Erhard