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

[Marco Elver]

On Mon, 7 Aug 2023 at 13:41, Florian Weimer <fweimer at redhat.com> 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:

I will add it back for the kernel documentation.

> | 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>

Good idea. I had already created
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899, and we need
better spec to proceed for GCC anyway.

> 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.

That's true, but is it a real problem? Isn't it known that trying to
make kernel modules built for a kernel with a different config (incl.
compiler) is not guaranteed to work? See IBT, CFI schemes, kernel
sanitizers, etc?

If we were to start trying to introduce some kind of minimal kernel to
module ABI so that modules and kernels built with different toolchains
keep working together, we'd need a mechanism to guarantee this minimal
ABI or prohibit incompatible modules and kernels somehow. Is there a
precedence for this somewhere?