Re: [PATCH 1 of 3] Remove use of bit fields in kvm trace structure

[Avi Kivity <avi@qumranet.com>]

Jerone Young wrote:
>
> This patch fixes kvmtrace use on big endian systems. When using bit fields the compiler will lay data out in the wrong order expected when laid down into a file. This fixes it by using one variable instead of using bit fields.
>
> Signed-off-by: Jerone Young <jyoung5@us.ibm.com>
>
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -311,9 +311,13 @@ struct kvm_s390_interrupt {
>  
>  /* This structure represents a single trace buffer record. */
>  struct kvm_trace_rec {
> -	__u32 event:28;
> -	__u32 extra_u32:3;
> -	__u32 cycle_in:1;
> +	/* variable rec_val
> + 	 * is split into:
> + 	 * bits 0 - 27  -> event id
> + 	 * bits 28 -30  -> number of extra data args of size u32
> + 	 * bits 31      -> binary indicator for if tsc is in record
> + 	 */
> +	__u32 rec_val;
>   

Please use #defines for shifts and masks instead of open-coding.  These 
#defines would be part of the public interface.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.