draft PR for gaining insight into what is needed for Go 1.17 calling conventions; DO NOT MERGE #104
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…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.
…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.
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For context, see the detailed commit messages of:
NOTE: this is a draft PR to gain intuition into the problem domain and is not intended to be commited into eggos. DO NOT MERGE.
NOTE: commit ef26ff1 is incompatible with Go versions prior to Go 1.17. In particular, it will not work with Go 1.16. A proper solution should handle both!
Updates #100.