[dpdk-dev] [PATCH] net: introduce IPv4 ihl and version fields

[Gregory Etelson]

> > > > RTE IPv4 header definition combines the `version' and `ihl'
> > > > fields into a single structure member.
> > > > This patch introduces dedicated structure members for both
> > `version'
> > > > and `ihl' IPv4 fields. Separated header fields definitions allow
> > > > to create simplified code to match on the IHL value in a flow rule.
> > > > The original `version_ihl' structure member is kept for backward
> > > > compatibility.
> > > >
> > > > Signed-off-by: Gregory Etelson <getelson at nvidia.com>
> > > > ---
> > > >  app/test/test_flow_classify.c |  8 ++++----
> > > >  lib/net/rte_ip.h              | 16 +++++++++++++++-
> > > >  2 files changed, 19 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/app/test/test_flow_classify.c
> > > > b/app/test/test_flow_classify.c index 951606f248..4f64be5357
> > > > 100644
> > > > --- a/app/test/test_flow_classify.c
> > > > +++ b/app/test/test_flow_classify.c
> > > > @@ -95,7 +95,7 @@ static struct rte_acl_field_def
> > > > ipv4_defs[NUM_FIELDS_IPV4] = {
> > > >   *  dst mask 255.255.255.00 / udp src is 32 dst is 33 / end"
> > > >   */
> > > >  static struct rte_flow_item_ipv4 ipv4_udp_spec_1 = {
> > > > - { 0, 0, 0, 0, 0, 0, IPPROTO_UDP, 0,
> > > > + { { .version_ihl = 0}, 0, 0, 0, 0, 0, IPPROTO_UDP, 0,
> > > >     RTE_IPV4(2, 2, 2, 3), RTE_IPV4(2, 2, 2, 7)}  };  static const
> > > > struct rte_flow_item_ipv4 ipv4_mask_24 = { @@ -131,7 +131,7 @@
> > > > static struct rte_flow_item  end_item = {
> RTE_FLOW_ITEM_TYPE_END,
> > > >   *  dst mask 255.255.255.00 / tcp src is 16 dst is 17 / end"
> > > >   */
> > > >  static struct rte_flow_item_ipv4 ipv4_tcp_spec_1 = {
> > > > - { 0, 0, 0, 0, 0, 0, IPPROTO_TCP, 0,
> > > > + { { .version_ihl = 0}, 0, 0, 0, 0, 0, IPPROTO_TCP, 0,
> > > >     RTE_IPV4(1, 2, 3, 4), RTE_IPV4(5, 6, 7, 8)}  };
> > > >
> > > > @@ -150,8 +150,8 @@ static struct rte_flow_item  tcp_item_1 = {
> > > > RTE_FLOW_ITEM_TYPE_TCP,
> > > >   *  dst mask 255.255.255.00 / sctp src is 16 dst is 17/ end"
> > > >   */
> > > >  static struct rte_flow_item_ipv4 ipv4_sctp_spec_1 = {
> > > > - { 0, 0, 0, 0, 0, 0, IPPROTO_SCTP, 0, RTE_IPV4(11, 12, 13, 14),
> > > > - RTE_IPV4(15, 16, 17, 18)}
> > > > + { { .version_ihl = 0}, 0, 0, 0, 0, 0, IPPROTO_SCTP, 0,
> > > > + RTE_IPV4(11, 12, 13, 14), RTE_IPV4(15, 16, 17, 18)}
> > > >  };
> > > >
> > > >  static struct rte_flow_item_sctp sctp_spec_1 = { diff --git
> > > > a/lib/net/rte_ip.h b/lib/net/rte_ip.h index 4b728969c1..684bb028b2
> > > > 100644
> > > > --- a/lib/net/rte_ip.h
> > > > +++ b/lib/net/rte_ip.h
> > > > @@ -38,7 +38,21 @@ extern "C" {
> > > >   * IPv4 Header
> > > >   */
> > > >  struct rte_ipv4_hdr {
> > > > - uint8_t  version_ihl;           /**< version and header length */
> > > > + __extension__
> > > > + union {
> > > > +         uint8_t version_ihl;    /**< version and header length */
> > > > +         struct {
> > > > +#if RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN
> > > > +                 uint8_t ihl:4;
> > > > +                 uint8_t version:4; #elif RTE_BYTE_ORDER ==
> > > > +RTE_BIG_ENDIAN
> > > > +                 uint8_t version:4;
> > > > +                 uint8_t ihl:4;
> > > > +#else
> > > > +#error "setup endian definition"
> > > > +#endif
> > > > +         };
> > > > + };
> > > >   uint8_t  type_of_service;       /**< type of service */
> > > >   rte_be16_t total_length;        /**< length of packet */
> > > >   rte_be16_t packet_id;           /**< packet ID */
> > > > --
> > > > 2.31.1
> > > >
> > >
> > > This does not break the ABI, but it could be discussed if it breaks
> > the API due to the required structure initialization changes shown in
> > > test_flow_classify.c.
> >
> > Yep, I guess it might be classified as API change.
> > Another thing that concerns me - it is not the only place in IPv4
> > header when we unite multiple bit-fields into one field:
> > type_of_service, fragment_offset.
> > If we start splitting ipv4 fields into actual bitfields, I suppose
> > we'll end-up splitting these ones too.
> > But I am not sure it will pay off - as compiler not always generates
> > optimal code for reading/updating bitfields.
> > Did you consider just adding extra macros to simplify access to these
> > fields (like RTE_IPV4_HDR_(GET_SET)_*), instead?
> >
> 
> Let's please not introduce accessor macros for bitfields. If we don't
> introduce bitfields like these, I would rather stick with the current _MASK,
> _SHIFT and _FLAG defines.
> 
> Yes, this change will lead to the introduction of more bitfields, both here
> and in other places. We already accepted it in the eCPRI structure
> (/lib/net/rte_ecpri.h), so why not just generally accept it.
> 
> Are modern compilers really worse at handling a bitfield defined like this,
> compared to handling a single uint8_t with hand coding? I consider your
> concern very important, so I'm only asking if it is still relevant, to avoid
> making decisions based on past experience that might be outdated. (I admit
> to falling into that trap myself, once in a while.)
> 

I compared x86 code generated with gcc-9, gcc-10 and clang-10 for these 2 functions:
void test_ipv4_hdr_byte(struct rte_ipv4_hdr *h, uint8_t version, uint8_t ihl)
{
	h->version_ihl = ((version & 0x0f) << 4) | (ihl & 0x0f);
}
void test_ipv4_hdr_bits(struct rte_ipv4_hdr *h, uint8_t version, uint8_t ihl)
{
	h->version = version & 0x0f;
	h->ihl = ihl & 0x0f;
}
meson configuration flags: --default-library=static --buildtype=release
Each compiler produced identical code for both functions. 
 

> > > I think this patch is an improvement, and that such structure
> > modifications should be generally accepted, so:
> > >
> > > Acked-by: Morten Brørup <mb at smartsharesystems.com>
> >