Re: Portable inline asm to get address of TLS variable

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Feb 17, 2022 at 04:02:16PM +0100, Serge Guelton wrote:
> On Thu, Feb 17, 2022 at 12:40:40PM +0100, Paolo Bonzini wrote:
> > On 2/17/22 10:28, Stefan Hajnoczi wrote:
> > >>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!
> > 
> > Yes, this is the kind of thing that I generally despise when I see
> > other programs do it...  it's easy to dig ourselves in the same
> > hole.
> > 
> > >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.
> > 
> > They're C++ only in GCC, too.  I really think that QEMU should be
> > compilable in C++, but I'm not sure how easy a sell it is.
> 
> It's perfectly fine to have one compilation unit written in C++ with a few
> symbol in `extern "C"`. No need to touch the other part of the project.
> 

I don't think that's possible in this case because the coroutine
functions are spread throughout the codebase. All coroutine functions
need to be in C++ source units so the compiler can transform them and
emit code callable as a coroutine.

Stefan

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