Portable inline asm to get address of TLS variable

[Stefan Hajnoczi]

Hi,
I've been trying to make the inline asm that gets the address of a TLS
variable for QEMU coroutines pass QEMU's GitLab CI.
https://gitlab.com/stefanha/qemu/-/blob/coroutine-tls-fix/include/qemu/coroutine-tls.h#L89

The code isn't -fPIC-friendly (R_X86_64_TPOFF32 relocations aren't
allowed in -fPIC shared libraries) so builds fail with ./configure
--enable-modules. While I was tackling this I stumbled on this:

  void *dst_ptr;
  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var))

What's nice about it:
- It's portable, there are no arch-specific assembly instructions.
- It works for both -fPIC and non-PIC.

However, I wonder if the compiler might reuse a register that already
contains the address. Then we'd have the coroutine problem again when
qemu_coroutine_yield() is called between the earlier address calculation
and the asm volatile statement.

Thoughts?

Stefan

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