Re: [PATCH] x86/elf: Remove ASM_CALL_CONSTRAINT from elf_core_save_regs()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2025 10:17, Andrew Cooper wrote:
> On 26/03/2025 9:00 am, Jan Beulich wrote:
>> On 25.03.2025 19:00, Andrew Cooper wrote:
>>> I was mistaken about when ASM_CALL_CONSTRAINT is applicable.  It is not
>>> applicable for plain pushes/pops, so remove it from the flags logic.
>>>
>>> Clarify the description of ASM_CALL_CONSTRAINT to be explicit about 
>>> unwinding
>>> using framepointers.
>>>
>>> Fixes: 0754534b8a38 ("x86/elf: Improve code generation in 
>>> elf_core_save_regs()")
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>>  xen/arch/x86/include/asm/asm_defns.h  | 5 +++--
>>>  xen/arch/x86/include/asm/x86_64/elf.h | 2 +-
>>>  2 files changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/include/asm/asm_defns.h 
>>> b/xen/arch/x86/include/asm/asm_defns.h
>>> index 92b4116a1564..689d1dcbf754 100644
>>> --- a/xen/arch/x86/include/asm/asm_defns.h
>>> +++ b/xen/arch/x86/include/asm/asm_defns.h
>>> @@ -28,8 +28,9 @@ asm ( "\t.equ CONFIG_INDIRECT_THUNK, "
>>>  
>>>  /*
>>>   * This output constraint should be used for any inline asm which has a 
>>> "call"
>>> - * instruction.  Otherwise the asm may be inserted before the frame pointer
>>> - * gets set up by the containing function.
>>> + * instruction, which forces the stack frame to be set up prior to the asm
>>> + * block.  This matters when unwinding using framepointers, where the asm's
>>> + * function can get skipped over.
>> Does "forces the stack frame to be set up" really mean the stack frame, or 
>> the
>> frame pointer (if one is in use)?
> 
> What do you consider to be the difference, given how frame pointers work
> in our ABI?

My point is that frame pointers are an optional part. Sufficiently high
optimization levels omit them by default, iirc. And depending on
CONFIG_FRAME_POINTER we may explicitly pass -fno-omit-frame-pointer. Even
in that case there is a stack frame that the compiler is setting up. Yet
in that case the effect of ASM_CALL_CONSTRAINT is not relevant. Hence
also why the construct expands to nothing in that case. The comment,
however, is placed outside if the #ifdef, and hence applies to both forms
(according to the way I read such, at least).

> It is the frame pointer which needs setting up, which at a minimum
> involves spilling registers to the stack and getting %rsp into it's
> eventual position.

Right, and all I'm effectively asking for is s/stack frame/frame pointer/
in the new comment text. Then:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Alternatively part or all of the comment could be moved inside the #ifdef.

Jan