Re: split lock issue in multiboot_dma.bin

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/7/2025 8:15 PM, Xiaoyao Li wrote:
> When running the kvm-unit-tests on Intel platforms with "split lock 
> disable" feature, every test triggers a kernel warning of
> 
>      x86/split lock detection: #AC: qemu-system-x86_64/373232 took a 
> split_lock trap at address: 0x1e3
> 
> After investigation, it turns out the split lock happens from 
> multiboot_dma.bin:
> 
> Hack KVM by exiting to QEMU on split lock #AC, we get
> 
> KVM: exception 17 exit (error code 0x0)
> EAX=00000001 EBX=00000000 ECX=00000014 EDX=0001fb80
> ESI=00000000 EDI=000000a8 EBP=00000000 ESP=00006f10
> EIP=000001e3 EFL=00010002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =0900 00009000 0000ffff 00009300 DPL=0 DS16 [-WA]
> CS =c000 000c0000 0000ffff 00009b00 DPL=0 CS16 [-RA]
> SS =0000 00000000 0000ffff 00009300 DPL=0 DS16 [-WA]
> DS =c000 000c0000 0000ffff 00009300 DPL=0 DS16 [-WA]
> FS =0950 00009500 0000ffff 00009300 DPL=0 DS16 [-WA]
> GS =06f2 00006f20 0000ffff 00009300 DPL=0 DS16 [-WA]
> LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
> TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
> GDT=     000c02b4 00000027
> IDT=     00000000 000003ff
> CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
> DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 
> DR3=0000000000000000
> DR6=00000000ffff0ff0 DR7=0000000000000400
> EFER=0000000000000000
> Code=89 16 08 00 65 66 0f 01 16 06 00 66 b8 01 00 00 00 0f 22 c0 <65> 66 
> ff 2e 00 00 b8 10 00 00 00 8e d0 8e d8 8e c0 8e e0 8e e8 66 b8 08 00 66 
> ba 10 05 66
> 
> And it matches with what disassembled from multiboo_dma.bin:
> 
> #objdump -b binary -m i386 -D pc-bios/multiboot_dma.bin
> 
>   1d1:   08 00                   or     %al,(%eax)
>   1d3:   65 66 0f 01 16          lgdtw  %gs:(%esi)
>   1d8:   06                      push   %es
>   1d9:   00 66 b8                add    %ah,-0x48(%esi)
>   1dc:   01 00                   add    %eax,(%eax)
>   1de:   00 00                   add    %al,(%eax)
>   1e0:   0f 22 c0                mov    %eax,%cr0
>  >1e3:   65 66 ff 2e             ljmpw  *%gs:(%esi)
>   1e7:   00 00                   add    %al,(%eax)
>   1e9:   b8 10 00 00 00          mov    $0x10,%eax
>   1ee:   8e d0                   mov    %eax,%ss
>   1f0:   8e d8                   mov    %eax,%ds
>   1f2:   8e c0                   mov    %eax,%es
>   1f4:   8e e0                   mov    %eax,%fs
>   1f6:   8e e8                   mov    %eax,%gs
>   1f8:   66 b8 08 00             mov    $0x8,%ax
>   1fc:   66 ba 10 05             mov    $0x510,%dx
> 
> I don't know where the multiboot_dma.bin are built from so that I cannot 
> further help resolve the split lock issue.

stupid me. The src code is just pc-bios/optionrom/multiboot.S
But I don't know how to build a bin file from it to test my fix.