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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev,
> Konstantin
> Sent: Friday, 28 May 2021 12.21
> 
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Gregory
> Etelson
> > > Sent: Thursday, 27 May 2021 17.29
> > and version fields
> > >
> > > 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@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 think this patch is an improvement, and that such structure
> modifications should be generally accepted, so:
> >
> > Acked-by: Morten Brørup <mb@smartsharesystems.com>
>