Commits on Feb 12, 2022

1. kernel: invoke sseInit and initFakeFS from rt0 to handle <autogenerat… …

   …ed> function wrappers of Go 1.17
   
   In Go 1.17, when invoking Go functions from asm, <autogenerated>
   function wrappers are generated to handle the conversion from the
   old Go ABI (abi0) which is stack based to the new internal
   register-based Go ABI (abi1).
   
   The problem is that the <autogenerated> function wrapper assumes
   that we are already running in a fully initialized Go environment,
   with support for SSE instructions and with a configured %fs segment
   register to handle TLS of G.
   
   For context, see the autogenerated function wrapper which is invoked
   when calling the Go kernel.preinit function from the asm rt0 function.
   
   	Dump of assembler code for function github.com/icexin/eggos/kernel.preinit<autogenerated>:
   		0x00000000002bbe40 <+0>:	sub    $0x18,%rsp
   		0x00000000002bbe44 <+4>:	mov    %rbp,0x10(%rsp)
   		0x00000000002bbe49 <+9>:	lea    0x10(%rsp),%rbp
   		0x00000000002bbe4e <+14>:	mov    0x20(%rsp),%rax
   		0x00000000002bbe53 <+19>:	mov    0x28(%rsp),%rbx
   	=> 0x00000000002bbe58 <+24>:	xorps  %xmm15,%xmm15
   		0x00000000002bbe5c <+28>:	mov    %fs:0xfffffffffffffff8,%r14
   		0x00000000002bbe65 <+37>:	call   0x2b6c40 <github.com/icexin/eggos/kernel.preinit>
   		0x00000000002bbe6a <+42>:	mov    0x10(%rsp),%rbp
   		0x00000000002bbe6f <+47>:	add    $0x18,%rsp
   		0x00000000002bbe73 <+51>:	ret
   
   As visible in the disassembly of kernel.preinit<autogenerated>,
   the XMM15 register is set to zero using XOR (see offset +24),
   which requires SSE instructions to be initialized.
   
   Also, on offset +28, the R14 register is moved into TLS by
   using the %fs segment register. R14 store G in Go 1.17.
   
   To handle this case, we need to initialize %fs to a valid memory
   location; we use a fake memory location, as no Go routines are
   running at this point, and later on %fs will be set when the first
   goroutine is started by thread0Init.
   
   Note, this is just a preliminary work to better understand what
   we must support to handle Go 1.17. We are most likely _not_ going
   to use this code for the final commit to eggos. This is just to
   get more intuition into the problem domain.
   
   Updates icexin#100.

   

   mewmew committed Feb 12, 2022
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2. kernel: skip return address of <autogenerated> function wrapper in tr… …

   …apret before IRET instruction
   
   NOTE: this commit will ONLY work for Go 1.17, it is not backwards
   compatible with Go 1.16. We should probably find a way to handle
   both (e.g. build tags or a general approach).
   
   To give context, an <autogenerated> function wrapper is generated
   in Go 1.17 for kernel.trapret.
   
   	Dump of assembler code for function github.com/icexin/eggos/kernel.trapret<autogenerated>:
   	=> 0x00000000002bbf60 <+0>:   call   0x2b9e40 <github.com/icexin/eggos/kernel.trapret>
   		0x00000000002bbf65 <+5>:   xorps  %xmm15,%xmm15
   		0x00000000002bbf69 <+9>:   mov    %fs:0xfffffffffffffff8,%r14
   		0x00000000002bbf72 <+18>:  ret
   
   In this autogenerated function, a call to the real kernel.trapret
   is made, which places a return address on the stack. However, the
   trapret function will use IRET to return, so to handle this case,
   we skip the return address pushed onto the stack by the autogenerated
   function by adding 8 (ptr size) to rsp.
   
   We should definitely try to find a better way to handle this situation
   as it seems very fragile and may break in the future if Go changes
   their autogenerated functions.
   
   So, once more, this is mostly to get intuition into the problem domain,
   it is not meant as a solution that should be merged into eggos.

   

   mewmew committed Feb 12, 2022
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