Re: [dpdk-dev] [RFC 4/4] net/ether: use bitops to speedup comparison

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 16 May 2019 17:36:43 +0100
Bruce Richardson <bruce.richardson@intel.com> wrote:

> On Thu, May 16, 2019 at 05:07:45PM +0100, Bruce Richardson wrote:
> > On Thu, May 16, 2019 at 09:06:52AM -0700, Stephen Hemminger wrote:  
> > > On Thu, 16 May 2019 17:03:37 +0100
> > > Bruce Richardson <bruce.richardson@intel.com> wrote:
> > >   
> > > > On Wed, May 15, 2019 at 03:19:52PM -0700, Stephen Hemminger wrote:  
> > > > > Using bit operations like or and xor is faster than a loop
> > > > > on all architectures. Really just explicit unrolling.
> > > > > 
> > > > > Similar cast to uint16 unaligned is already done in
> > > > > other functions here.
> > > > > 
> > > > > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > > > > ---
> > > > >  lib/librte_net/rte_ether.h | 17 +++++++----------
> > > > >  1 file changed, 7 insertions(+), 10 deletions(-)
> > > > >     
> > > > Rather than casting to unaligned values, which gives compiler warnings in
> > > > some cases, I believe we should just mark the ethernet addresses as always
> > > > being 2-byte aligned and simplify things. [unless we have a good use case
> > > > where we won't have 2-byte alignment???].
> > > > 
> > > > See patch: http://patches.dpdk.org/patch/53482/
> > > > 
> > > > Regards,
> > > > /Bruce  
> > > 
> > > I agree. Then you could also remove the unaligned_uint16_t that
> > > already exists in rte_ether.h
> > > 
> > > Do you want me to put your patch in my series?  
> > 
> > Sure, feel free.
> >   
> 
> I've just found another problem in this area. Compiling l2fwd with gcc 9, I
> get:
> 
> main.c:164:54: warning: taking address of packed member of ‘struct ether_hdr’ may result in an unaligned pointer value [-Waddress-of-packed-member]
>   164 |  ether_addr_copy(&l2fwd_ports_eth_addr[dest_portid], &eth->s_addr);
>       | 
> 
> Looking at some of the structures, it appears that not all the packet
> attributes may be necessary. For example, while AFAIK there are not
> absolute guarantees about structure padding, for all compilers I remember
> using the following changes are safe:
> 
> diff --git a/lib/librte_net/rte_ether.h b/lib/librte_net/rte_ether.h
> index 8090b7c01..7d9f34791 100644
> --- a/lib/librte_net/rte_ether.h
> +++ b/lib/librte_net/rte_ether.h
> @@ -57,7 +57,7 @@ extern "C" {
>  struct ether_addr {
>         /** Addr bytes in tx order */
>         uint8_t addr_bytes[ETHER_ADDR_LEN] __rte_aligned(2);
> -} __attribute__((__packed__));
> +};
> 
>  #define ETHER_LOCAL_ADMIN_ADDR 0x02 /**< Locally assigned Eth. address. */
>  #define ETHER_GROUP_ADDR       0x01 /**< Multicast or broadcast Eth. address. */
> @@ -273,7 +273,7 @@ struct ether_hdr {
>         struct ether_addr d_addr; /**< Destination address. */
>         struct ether_addr s_addr; /**< Source address. */
>         uint16_t ether_type;      /**< Frame type. */
> -} __attribute__((__packed__));
> +};
> 
>  /**
>   * Ethernet VLAN Header.
> @@ -283,7 +283,7 @@ struct ether_hdr {
>  struct vlan_hdr {
>         uint16_t vlan_tci; /**< Priority (3) + CFI (1) + Identifier Code (12) */
>         uint16_t eth_proto;/**< Ethernet type of encapsulated frame. */
> -} __attribute__((__packed__));
> +};
> 
>  /**
>   * VXLAN protocol header.
> 
> I think we therefore should consider removing those packed attributes to
> avoid application warnings.
> 
> /Bruce

Agree if structure is naturally packed, adding packed attribute is a mistake.