[llvm-dev] Thread-local-storage memory layout
Anatol Pomozov via llvm-dev
llvm-dev at lists.llvm.org
Sat Jan 27 10:20:33 PST 2018
I am writing a freestanding statically linked x86_64 kernel. And now I
want to load and use thread-local storage. As I have a freestanding
binary I can't use libc/Linux kernel thread-local mechanism and need
to implement initialization code by myself.
I am trying to find a clear explanation of what is the memory layout
for x86 TLS but can't find anything useful. Maybe LLVM developers who
worked with TLS could help me.
Ok, I have two TLS sections .tdata .tbss
[ 6] .tdata PROGBITS 00000000002a90d0 001a5dd0
0000000000000004 0000000000000000 WAT 0 0 16
[ 7] .tbss NOBITS 00000000002a90e0 001a5dd4
0000000000009c94 0000000000000000 WAT 0 0 16
And here is the ELF segment information:
TLS 0x00000000001a5dd0 0x00000000002a90d0 0x00000000002a90d0
0x0000000000000004 0x0000000000009ca4 R 0x10
So I load this segment into memory using file offset, filesize, memorysize.
My understanding that for x86_64 there is a "struct thread_info" that
is located right after TLS loaded segment. And the first element of
this structure is a pointer (i.e. sizeof(uintptr_t)).
Now I need to setup %fs and thread_info structure and it is where I
have a few questions:
* what is the alignment requirement for struct thread_info?. Is it
sizeof(uintptr_t) (i.e. 8 bytes at x86_64)?
* where thread_info->pointer points to? Does it point to the struct itself?
* what should be value of %fs register? Is it address of thread_info struct?
Thank you in advance.
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