Re: [PATCH 1/2] ArmPkg/AsmMacroIoLibV8: Introduce ASM_FUNC_ALIGN()

["Leif Lindholm" <quic_llindhol@quicinc.com>]

Hi Marvin,

First of all - many thanks for tracking down the bug that creates the
need for this.

On Mon, Apr 17, 2023 at 18:09:15 +0000, Marvin Häuser wrote:
> With the current ASM_FUNC() macro, there is no good way to declare an
> alignment constraint for a function. As ASM_FUNC() switches sections,
> declaring the constraint before the macro invocation applies it to the
> current location in the previous section. Declaring the constraint after
> the macro invocation lets the function label point to the location prior
> to alignment. Depending on toolchain behaviour, this may cause the label
> to point to alignment padding preceding the actual function definition.
> 
> To address these issues, introduce the ASM_FUNC_ALIGN() macro, which
> declares the alignment constraint right before the function label.
> 
> Signed-off-by: Marvin Häuser <mhaeuser@posteo.de>
> Cc: Leif Lindholm <quic_llindhol@quicinc.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Sami Mujawar <sami.mujawar@arm.com>
> Cc: Vitaly Cheptsov <vit9696@protonmail.com>
> ---
>  ArmPkg/Include/AsmMacroIoLibV8.h | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/ArmPkg/Include/AsmMacroIoLibV8.h b/ArmPkg/Include/AsmMacroIoLibV8.h
> index 135aaeca5d0b..919edc70384d 100644
> --- a/ArmPkg/Include/AsmMacroIoLibV8.h
> +++ b/ArmPkg/Include/AsmMacroIoLibV8.h
> @@ -34,15 +34,29 @@
>          cbnz   SAFE_XREG, 1f        ;\
>          b      .                    ;// We should never get here
>  
> -#define _ASM_FUNC(Name, Section)    \
> -  .global   Name                  ; \
> -  .section  #Section, "ax"        ; \
> -  .type     Name, %function       ; \
> +#define _ASM_FUNC_HDR(Name, Section) \
> +  .global   Name                   ; \
> +  .section  #Section, "ax"         ; \
> +  .type     Name, %function
> +
> +#define _ASM_FUNC_FTR(Name)         \
>    Name:                           ; \
>    AARCH64_BTI(c)
>  
> +#define _ASM_FUNC(Name, Section)    \
> +  _ASM_FUNC_HDR(Name, Section)    ; \
> +  _ASM_FUNC_FTR(Name)
> +
> +#define _ASM_FUNC_ALIGN(Name, Section, Align)       \

I like this solution, but I'd like to hear Ard's opinion.

I probably want to bikeshed some of the implementation details:
Although I generally dislike duplicate definitions, I think I would
prefer having _ASM_FUNC and _ASM_FUNC_ALIGN defined self-contained,
without _HDR and _FTR.
If we do keep the reused primitives, we need better language; the
footer of the header is not a footer of the function.

/
    Leif

> +  _ASM_FUNC_HDR(Name, Section)                    ; \
> +  .balign Align                                   ; \
> +  _ASM_FUNC_FTR(Name)
> +
>  #define ASM_FUNC(Name)  _ASM_FUNC(ASM_PFX(Name), .text. ## Name)
>  
> +#define ASM_FUNC_ALIGN(Name, Align)  \
> +  _ASM_FUNC_ALIGN(ASM_PFX(Name), .text. ## Name, Align)
> +
>  #define MOV32(Reg, Val)                   \
>    movz      Reg, (Val) >> 16, lsl #16   ; \
>    movk      Reg, (Val) & 0xffff
> -- 
> 2.40.0
>