Re: Portable inline asm to get address of TLS variable

[Paolo Bonzini]

On 2/16/22 18:46, Stefan Hajnoczi wrote:
> 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.

Yes, the compiler should be able to reuse the register.  volatile only 
says that the contents of the "asm" cannot be subject to e.g. loop 
optimizations:

        for (i = 0; i < 10; i++) {
                asm("# assembly": "=r"(k) : "0"(10));
                j += k;
        }

will likely execute the asm once, while "asm volatile" (or an asm 
without inputs, which is always volatile) will execute it ten times.

However, the input of the assembly can be evaluated only once either 
way.  For example, in the case above you might have "movl $10, %edx" 
outside the loop even with asm volatile.

One way to fix it for modules could be to define a (global, non-TLS) 
variable in QEMU with the %fs-based offset of the relevant thread-local 
variable, and initialize it before modules are loaded.

Paolo