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<damo22>ok i registered the boot data in biosmem and got a console log now

<damo22>the remaining issue is GS

<damo22>Breakpoint 2, cpu_launch_first_thread (th=<optimized out>) at ../kern/startup.c:320

<damo22>320 load_context(th);

<damo22>up until here, GS is set correctly

<damo22>when i execute that function in gdb GS gets corrupt

<damo22>and it double faults

<damo22>switch_ktss breaks it

<damo22>where are the segments switched?

<damo22>aha

<damo22>percpu_assign() is called after the gs is reloaded

<damo22>you cant switch to the new thread and THEN update the percpu active thread using gs

<damo22>youpi: do kernel threads run as a "normal" threads? if so, dont we need to populate gs base address in the thread state for the kernel threads so they dont get erased?

<damo22> 11013: 0f 01 16 32 11 lgdtw 0x1132

<damo22>...

<damo22> 1102a: ea 2f 10 08 00 ljmp $0x8,$0x102f

<damo22>is that the correct offset or should it be 0x2f

<damo22>ahh i see you can do a 16 bit ljmp with 32 bit immediate

<damo22>but GAS has trouble assembling it

<damo22>66 ea xx xx xx xx yy yy

<damo22>0F 01 /2 LGDT m16&32

<damo22>what does the /2 mean in the op code description?

<damo22>ok

<damo22>looks like lgdt can only take a 16 bit immediate in real mode

<damo22>but the descriptor can contain a 32 bit address

<damo22>got to protected mode on AP!

<youpi>damo22: I don't know what you call a "normal" thread

<youpi>during my couple decade of working on parallel computing, I have seen so many different kinds of "threads" :)

<youpi>about gs/fs, we don't use them for implementing some __thread per-thread data

<youpi>we use gs for per-cpu data, see percpu.h

<damo22> 0x0001103c <+10>: 0f 01 15 72 5c 09 01 lgdtl 0x1095c72

<damo22> 0x00011043 <+17>: ea 4a 10 01 00 08 00 ljmp $0x8,$0x1104a

<damo22> 0x0001104a <+24>: b8 ef be ad de mov $0xdeadbeef,%eax

<damo22> 0x0001104f <+29>: f4 hlt

<damo22>^^^ the only conclusion i can draw is that the ljmp is wrongly specified because the AP executes and loads the gdt but it never executes the halt and runs away somewhere else

<damo22>CS =0008 40000000 ffffffff 00c09b00 DPL=0 CS32 [-RA]

<damo22>GDT= 01095c80 0000006f

<azert>damo22: what is the segment descriptor? If you are calling into a call gate, the offset is ignored

<damo22>it seems like my code segment is not executable??

<damo22>is that posslbe

IRC channel logs

2025-12-31.log