linux-kernel - Re: [tip:x86/asm] x86: document 64-bit and 32-bit function call convention ABI

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Date:	Tue, 3 Feb 2009 19:48:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [tip:x86/asm] x86: document 64-bit and 32-bit function call
	convention ABI


* Randy Dunlap <randy.dunlap@...cle.com> wrote:

> Ingo Molnar wrote:
> > - also clean up the calling.h file a tiny bit
> > 
> > Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > ---
> >  arch/x86/include/asm/calling.h |   56 +++++++++++++++++++++++++++++++++++++--
> >  1 files changed, 53 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
> > index 2bc162e..2d36d42 100644
> > --- a/arch/x86/include/asm/calling.h
> > +++ b/arch/x86/include/asm/calling.h
> > @@ -1,5 +1,55 @@
> >  /*
> > - * Some macros to handle stack frames in assembly.
> > +
> > + x86 function call convention, 64-bit:
> > + -------------------------------------
> > +  arguments           |  callee-saved      | extra caller-saved | return
> > + [callee-clobbered]   |                    | [callee-clobbered] |
> > + ---------------------------------------------------------------------------
> > + rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11             | rax, rdx [**]
> > +
> > + ( rsp is obviously invariant across normal function calls. (gcc can 'merge'
> > +   functions when it sees tail-call optimization possibilities) rflags is
> > +   clobbered. Leftover arguments are passed over the stack frame.)
> > +
> > + [*]  In the frame-pointers case ebp is fixed to the stack frame.
> 
>                                     ebp or rbp ?

rbp - fixed it, thanks.

> > +
> > + [**] for struct return values wider than 64 bits the return convention is a
> > +      bit more complex: up to 128 bits width we return small structures
> > +      straight in rax, rdx. For structures larger than that (3 words or
> 
> 	What size does "word" mean here?
> 	and is it rdx:rax (high:low)?  Can it be so written?
> 	or at least say which of rax, rdx is high (most significant) and
> 	which is low (least significant).

no, word is the normal meaning: natural machine word. 32-bit on 32-bit x86, 
64-bit on 64-bit x86.

> 
> > +      larger) the caller puts a pointer to an on-stack return struct
> > +      [allocated in the caller's stack frame] into the first argument - i.e.
> > +      into rdi. All other arguments shift up by one in this case.
> > +      Fortunately this case is rare in the kernel.
> > +
> > +For 32-bit we have the following conventions - kernel is build with
> 
>                                                             built

fixed that too.

> > +-mregparm=3 and -freg-struct-return:
> > +
> > + x86 function calling convention, 32-bit:
> > + ----------------------------------------
> > +  arguments         | callee-saved        | extra caller-saved | return
> > + [callee-clobbered] |                     | [callee-clobbered] |
> > + -------------------------------------------------------------------------
> > + eax edx ecx        | ebx edi esi ebp [*] | <none>             | eax, edx [**]
> > +
> > + ( here too esp is obviously invariant across normal function calls. eflags
> > +   is clobbered. Leftover arguments are passed over the stack frame. )
> > +
> > + [*]  In the frame-pointers case ebp is fixed to the stack frame.
> > +
> > + [**] We build with -freg-struct-return, which on 32-bit means similar
> > +      semantics as on 64-bit: edx can be used for a second return value
> > +      (i.e. covering integer and structure sizes up to 64 bits) - after that
> > +      it gets more complex and more expensive: 3-word or larger struct returns
> > +      get done in the caller's frame and the pointer to the return struct goes
> > +      into regparm0, i.e. eax - the other arguments shift up and the
> > +      function's register parameters degenerate to regparm=2 in essence.
> > +
> 
> 	Same high:low comments here.
> 
> > +*/
> > +
> > +
> > +/*
> > + * 64-bit system call stack frame layout defines and helpers,
> > + * for assembly code:
> >   */
> >  
> >  #define R15		  0
> > @@ -9,7 +59,7 @@
> >  #define RBP		 32
> >  #define RBX		 40
> >  
> > -/* arguments: interrupts/non tracing syscalls only save upto here*/
> > +/* arguments: interrupts/non tracing syscalls only save upto here: */
> 
>                                                            up to

pre-existing typo but worth fixing indeed.

	Ingo
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