Re: Portable inline asm to get address of TLS variable

[Florian Weimer <fweimer@redhat.com>]

* Stefan Hajnoczi:

>> But going against ABI and toolchain in this way is really no long-term
>> solution.  You need to switch to stackless co-routines, or we need to
>> provide proper ABI-level support for this.  Today it's the thread
>> pointer, tomorrow it's the shadow stack pointer, and the day after that,
>> it's the SafeStack pointer.  And further down the road, it's some thread
>> state for garbage collection support.  Or something like that.
>
> Yes, understood :(. This does feel like solving an undefined behavior
> problem by adding more undefined behavior on top!
>
> Stackless coroutines have been tried in the past using Continuation
> Passing C (https://github.com/kerneis/cpc). Ideally we'd use a solution
> built into the compiler though. I'm concerned that CPC might not be
> supported or available everywhere QEMU needs to run now and in the
> future.

That seems to be require an entirely different toolchain (based on CIL).
It's one way to solve the ABI issues, but perhaps not the direction
you want to go in.

> I took a quick look at C++20 coroutines since they are available in
> compilers but the primitives look hard to use even from C++, let alone
> from C.

Could you go into details what makes them hard to use?  Is it because
coroutines are infectious across the call stack?

Thanks,
Florian