Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

[Peter Zijlstra]

On Tue, Apr 05, 2016 at 06:02:25PM +0200, Florian Weimer wrote:
> On 04/04/2016 10:48 PM, Mathieu Desnoyers wrote:
> 
> > Moreover, the feature set that the application knows about, glibc
> > knows about, and the kernel knows about are three different things.
> > My intent here is to have glibc stay out of the way as much as possible,
> > since this is really an interface between various applications/libraries
> > and the kernel.
> 
> Surely glibc can allocate the space based on what is advertised as
> needed by the kernel?  Why would it limit itself to what is supported by
> the kernel headers it is compiled against if the actual size can be
> queried from the kernel?

I guess the question is; can we do thread local variable arrays like:

  __thread uint32_t[x]; /* with x being a runtime constant */

Because then we can do:

  __thread struct thread_local_abi tla;

where sizeof(struct thread_local_abi) is a runtime variable.

Without that we cannot have this thread-local-abi structure be part of
the immediately addressable TLS space. That is, we then need a pointer
like:

  __thread struct thread_local_abi *tla;

and every usage will need the extra pointer deref.

Because ideally this structure would be part of the initial (glibc) TCB
with fixed offset etc.