[PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

[Peter Zijlstra]

On Mon, Aug 07, 2023 at 01:41:07PM +0200, Florian Weimer wrote:
> * Marco Elver:
> 
> > [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> > convention of a function. The preserve_most calling convention attempts
> > to make the code in the caller as unintrusive as possible. This
> > convention behaves identically to the C calling convention on how
> > arguments and return values are passed, but it uses a different set of
> > caller/callee-saved registers. This alleviates the burden of saving and
> > recovering a large register set before and after the call in the
> > caller."
> >
> > [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> 
> You dropped the interesting part:
> 
> | If the arguments are passed in callee-saved registers, then they will
> | be preserved by the callee across the call. This doesn’t apply for
> | values returned in callee-saved registers.
> | 
> |  ·  On X86-64 the callee preserves all general purpose registers, except
> |     for R11. R11 can be used as a scratch register. Floating-point
> |     registers (XMMs/YMMs) are not preserved and need to be saved by the
> |     caller.
> |     
> |  ·  On AArch64 the callee preserve all general purpose registers, except
> |     X0-X8 and X16-X18.
> 
> Ideally, this would be documented in the respective psABI supplement.
> I filled in some gaps and filed:
> 
>   Document the ABI for __preserve_most__ function calls
>   <https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/45>
> 
> Doesn't this change impact the kernel module ABI?
> 
> I would really expect a check here
> 
> > +#if __has_attribute(__preserve_most__)
> > +# define __preserve_most notrace __attribute__((__preserve_most__))
> > +#else
> > +# define __preserve_most
> > +#endif
> 
> that this is not a compilation for a module.  Otherwise modules built
> with a compiler with __preserve_most__ attribute support are
> incompatible with kernels built with a compiler without that attribute.

We have a metric ton of options that can break module ABI. If you're
daft enough to not build with the exact same compiler and .config you
get to keep the pieces.