Messages in this thread | | | Date | Mon, 7 Aug 2023 14:31:37 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute |
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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.
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