Re: [PATCH RFC bpf-next 05/20] trait: Replace memcpy calls with inline copies

[David Laight <david.laight.linux@gmail.com>]

On Wed, 05 Mar 2025 15:32:02 +0100
arthur@arthurfabre.com wrote:

> From: Arthur Fabre <afabre@cloudflare.com>
> 
> When copying trait values to or from the caller, the size isn't a
> constant so memcpy() ends up being a function call.
> 
> Replace it with an inline implementation that only handles the sizes we
> support.
> 
> We store values "packed", so they won't necessarily be 4 or 8 byte
> aligned.
> 
> Setting and getting traits is roughly ~40% faster.
> 
> Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
> ---
>  include/net/trait.h | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/include/net/trait.h b/include/net/trait.h
> index 536b8a17dbbc091b4d1a4d7b4b21c1e36adea86a..d4581a877bd57a32e2ad032147c906764d6d37f8 100644
> --- a/include/net/trait.h
> +++ b/include/net/trait.h
> @@ -7,6 +7,7 @@
>  #include <linux/errno.h>
>  #include <linux/string.h>
>  #include <linux/bitops.h>
> +#include <linux/unaligned.h>
>  
>  /* Traits are a very limited KV store, with:
>   * - 64 keys (0-63).
> @@ -145,23 +146,23 @@ int trait_set(void *traits, void *hard_end, u64 key, const void *val, u64 len, u
>  			memmove(traits + off + len, traits + off, traits_size(traits) - off);
>  	}
>  
> -	/* Set our value. */
> -	memcpy(traits + off, val, len);
> -
> -	/* Store our length in header. */
>  	u64 encode_len = 0;
> -
>  	switch (len) {
>  	case 2:
> +		/* Values are least two bytes, so they'll be two byte aligned */
> +		*(u16 *)(traits + off) = *(u16 *)val;
>  		encode_len = 1;
>  		break;
>  	case 4:
> +		put_unaligned(*(u32 *)val, (u32 *)(traits + off));
>  		encode_len = 2;
>  		break;
>  	case 8:
> +		put_unaligned(*(u64 *)val, (u64 *)(traits + off));
>  		encode_len = 3;
>  		break;
>  	}
> +
>  	h->high |= (encode_len >> 1) << key;
>  	h->low |= (encode_len & 1) << key;
>  	return 0;
> @@ -201,7 +202,19 @@ int trait_get(void *traits, u64 key, void *val, u64 val_len)
>  	if (real_len > val_len)
>  		return -ENOSPC;
>  
> -	memcpy(val, traits + off, real_len);
> +	switch (real_len) {
> +	case 2:
> +		/* Values are least two bytes, so they'll be two byte aligned */
> +		*(u16 *)val = *(u16 *)(traits + off);
> +		break;
> +	case 4:
> +		*(u32 *)val = get_unaligned((u32 *)(traits + off));
> +		break;
> +	case 8:
> +		*(u64 *)val = get_unaligned((u64 *)(traits + off));
> +		break;

Should there be a 'default' in here?
Possibly just 'return 0'?

> +	}
> +
>  	return real_len;
>  }
>  
>