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SubjectRe: [LKP] Re: [x86/mm/64] f154f29085: BUG:kernel_reboot-without-warning_in_boot_stage
Hi Boris,

On 12/15/2021 6:05 PM, Borislav Petkov wrote:
> On Wed, Dec 15, 2021 at 03:00:13PM +0800, Carel Si wrote:
>> We have verified by hand, still can reproduce this issue.
> Ok, please give details how exactly you reproduce: host, guest, kernel
> versions, configs, machine types, i.e., /proc/cpuinfo, dmesg, etc. I'd
> like to see if I can find a similar machine here.
> Also, would it be possible to upload your vmlinuz somewhere so that I
> can download it for testing?

The testing was with Qemu. And we found that the hang is related with

The original report showed the kernel is built with clang-14:
# build kernel
cd linux
cp config-5.16.0-rc3-00003-gf154f290855b .config
make HOSTCC=clang-14 CC=clang-14 ARCH=x86_64 olddefconfig prepare modules_prepare bzImage modules
make HOSTCC=clang-14 CC=clang-14 ARCH=x86_64 INSTALL_MOD_PATH=<mod-install-dir> modules_install

And the clang-14 generate different code comparing to clang-11. I pasted
the native_write_cr4 assembly code generated with clang-14 and clang-11 to:

The extra code generated by clang-14 is like:

ffffffff810b8784: 48 89 d8 mov %rbx,%rax
ffffffff810b8787: 48 c1 e8 03 shr $0x3,%rax
ffffffff810b878b: 48 b9 00 00 00 00 00 movabs $0xdffffc0000000000,%rcx
ffffffff810b8792: fc ff df
ffffffff810b8795: 80 3c 08 00 cmpb $0x0,(%rax,%rcx,1)
--> Qemu reboot after this instruction from x86_64_start_kernel
ffffffff810b8799: 74 08 je ffffffff810b87a3 <native_write_cr4+0x84>
ffffffff810b879b: 48 89 df mov %rbx,%rdi
ffffffff810b879e: e8 cc 7c 64 00 callq ffffffff8170046f <__asan_report_load8_noabort>
ffffffff810b87a3: 48 ff 03 incq (%rbx)
ffffffff810b87a6: 5b pop %rbx

Looks like KASAN related stub generated by clang-14 (KASAN_SHADOW_OFFSET and asan_report).
This function is early function called before kasan_init.

Looks like we need to disable KASAN_SANITIZE for arch/x86/kernel/cpu/common.c. So clang-14 will
be happy with this kind of early TLB flush? Thanks.

Yin, Fengwei

> Thx.

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