Ping: [XEN PATCH] xen: add paddings after bitmap section in LLVM coverage profile

[Wentao Zhang <zhangwt1997@xxxxxxxxx>]

Thanks,
Wentao 

On Sat, 20 Dec 2025 05:22:43 -0600, Wentao Zhang <zhangwt1997@xxxxxxxxx> wrote:
> The layout of LLVM coverage profile is like
> 
>   header
>   data section
>   (padding #1)
>   counter section
>   (padding #2)
>   bitmap section
>   (padding #3)
>   name section
>   (padding #4)
> 
> Padding areas #1 and #2 are always zeroed on 64-bit platforms, but that
> is not the case for padding area #3 and #4. See LLVM docs [1] and
> compiler-rt's own version of "get_size()" [2].
> 
> The implementation in 08c787f "xen: Enable MC/DC coverage for Clang"
> partly considers padding #4 in get_size() but not in dump(). It worked
> because in the header .padding_bytes_after_bitmap_bytes is also
> initialized to zero so a reader may still know how to parse the profile.
> But we should probably not base ourselves on such assumption. Instead
> let's be as close as possible to hosted environment generated profiles,
> i.e. those generated by compiler-rt.
> 
> In this patch, get_size() implementation is mathematically the same but
> changed to reflect the layout somewhat better. For dump(), padding #4 is
> added both in the header and in the payload.
> 
> [1] https://llvm.org/docs/InstrProfileFormat.html
> [2] 
> https://github.com/llvm/llvm-project/blob/llvmorg-20.1.8/compiler-rt/lib/profile/InstrProfilingBuffer.c#L223
> 
> Signed-off-by: Wentao Zhang <zhangwt1997@xxxxxxxxx>
> 
> ---
> 
> As an aside, an alternative way that has better long-term
> maintainability would be [3]. I ran it with Xen and could unofficially
> confirm it works, modulo implementation nitty-gritties.
> 
> [3] https://github.com/llvm/llvm-project/pull/167998
> ---
>  xen/common/coverage/llvm.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/common/coverage/llvm.c b/xen/common/coverage/llvm.c
> index 5663fb1..f15ec11 100644
> --- a/xen/common/coverage/llvm.c
> +++ b/xen/common/coverage/llvm.c
> @@ -141,11 +141,11 @@ static void cf_check reset_counters(void)
>  
>  static uint32_t cf_check get_size(void)
>  {
> -    uint32_t size = ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - 
> START_DATA +
> -                   END_COUNTERS - START_COUNTERS + END_NAMES - START_NAMES, 
> 8);
> -    if ( IS_ENABLED(CONFIG_CONDITION_COVERAGE) )
> -        size += ROUNDUP(END_BITMAP - START_BITMAP, 8);
> -    return size;
> +    return sizeof(struct llvm_profile_header) +
> +           END_DATA - START_DATA +
> +           END_COUNTERS - START_COUNTERS +
> +           ROUNDUP(END_BITMAP - START_BITMAP, 8) +
> +           ROUNDUP(END_NAMES - START_NAMES, 8);
>  }
>  
>  static int cf_check dump(
> @@ -167,6 +167,7 @@ static int cf_check dump(
>  #if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9
>          .num_bitmap_bytes = END_BITMAP - START_BITMAP,
>          .bitmap_delta = START_BITMAP - START_DATA,
> +        .padding_bytes_after_bitmap_bytes = (-(END_BITMAP - START_BITMAP)) & 
> 7,
>  #endif
>      };
>      unsigned int off = 0;
> @@ -183,6 +184,7 @@ static int cf_check dump(
>      APPEND_TO_BUFFER(START_COUNTERS, END_COUNTERS - START_COUNTERS);
>  #if defined(CONFIG_CONDITION_COVERAGE)
>      APPEND_TO_BUFFER(START_BITMAP, END_BITMAP - START_BITMAP);
> +    off += header.padding_bytes_after_bitmap_bytes;
>  #endif
>      APPEND_TO_BUFFER(START_NAMES, END_NAMES - START_NAMES);
>  #undef APPEND_TO_BUFFER
> -- 
> 2.34.1