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

[Sandipan Das <sandipan.das@amd.com>]

On 1/19/2023 8:48 PM, Peter Zijlstra wrote:
> On Thu, Jan 19, 2023 at 02:56:08PM +0100, Peter Zijlstra wrote:
>> On Thu, Jan 19, 2023 at 02:23:03AM +0100, Erhard F. wrote:
>>> Hi!
>>>
>>> I did a kernel bisect for an issue I originally posted on https://github.com/ClangBuiltLinux/linux/issues/1774
>>>
>>> It is about kernel 6.1.x not booting on my machines when built with clang. A gcc built kernel with the same config just works fine. Turns out kernel v6.2-rc4 and earlier v6.2-rc are still affected.
>>>
>>> I did a kernel bisect which revealed this commit:
>>>
>>>  # git bisect bad
>>> 706460a96fc654e80b6bed1f562b00d2ce9f2f4d is the first bad commit
>>> commit 706460a96fc654e80b6bed1f562b00d2ce9f2f4d
>>> Author: Sandipan Das <sandipan.das@amd.com>
>>> Date:   Thu Aug 11 17:59:51 2022 +0530
>>>
>>>     perf/x86/amd/core: Add generic branch record interfaces
>>>     
>>>     AMD processors that are capable of recording branches support either Branch
>>>     Sampling (BRS) or Last Branch Record (LBR). In preparation for adding Last
>>>     Branch Record Extension Version 2 (LbrExtV2) support, introduce new static
>>>     calls which act as gateways to call into the feature-dependent functions
>>>     based on what is available on the processor.
>>>     
>>>     Signed-off-by: Sandipan Das <sandipan.das@amd.com>
>>>     Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>>     Link: https://lore.kernel.org/r/b75dbc32663cb395f0d701167e952c6a6b0445a3.1660211399.git.sandipan.das@amd.com
>>>
>>>  arch/x86/events/amd/core.c | 34 ++++++++++++++++++++++------------
>>>  1 file changed, 22 insertions(+), 12 deletions(-)
>>
>> Using: v6.2-rc4-67-g7287904c8771
>>
>> I have an AMD Interlagos and after bringing it back to live/up-to-date I
>> can't seem to even boot your .config in qemu/kvm irrespective of
>> GCC/Clang.
>>

I had a similar experience on a system with an AMD EPYC 7713. Both guest
and bare-metal boot failed.

>> When I build defconfig+kvm_guest.config both GCC-12 and clang-15 boot
>> just fine in qemu/kvm.
> 
> Furthermore, testing localyesconfig based off Debian's
> config-6.1.0-1-amd64 boots fine using both GCC-12 and clang-15 as host
> kernels on that machine.
> 

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.

- Sandipan