From: ext Bill Fischofer [mailto:[email protected]] Sent: Wednesday, December 17, 2014 2:40 PM To: Savolainen, Petri (NSN - FI/Espoo) Cc: Petri Savolainen; lng-odp-forward Subject: Re: [lng-odp] [PATCH] Remove packet l2/l3/l4 offset adjustments
While ODP may not know why an application is choosing to make a specific push/pull request, the implications of any given call are precise. After pushing the head by X bytes, all of the other packet offset metadata are now in error until and unless they are adjusted. If it is part of the API definition that the implementation MUST NOT assist in these adjustments, then there are several implications: 1. Other ODP APIs that reference these now-erroneous metadata are invalidated and will silently return incorrect values. Does this help promote good programming? We have to specify (per ODP API) anyway which metadata fields are required to have valid values. If application creates a packet and passes it to e.g. a crypto IPSEC API, it has to first fill in the correct metadata (e.g. l3 offset). 2. The application takes responsibility for knowing which metadata items have been invalidated, even if additional metadata is added to ODP in subsequent releases. How does it know this? It’s then a new API version. An application is coded against an API version. 3. As has been pointed out, since the API does not provide relative offset adjustment APIs, for an application to adjust a given offset value correctly, it must read/modify/write each individually. Hardly efficient. Application likely read the offsets anyway before push/pull. Also that data is likely on same cache with all other packet metadata. There’s really no difference if application increments offsets by ‘len’ or if implementation does it. The real difference is that is an offset is written twice vs only once, OR once vs not at all. 4. Implementations that support push/pull operations with HW assistance may need to add special code to undo any automatic metadata adjustments to comply with the API. Is there any HW like this? Typically these are just fields in the packet descriptor (in the memory) and HW does not see it before application passes the packet to e.g. the output HW. -Petri So it's not at all clear how the application benefits from this stipulation or how it would be harmed if it was the implementations responsibility to maintain the system metadata in a consistent state. On Wed, Dec 17, 2014 at 6:24 AM, Savolainen, Petri (NSN - FI/Espoo) <[email protected]<mailto:[email protected]>> wrote: Once again. There’s no point of implementation speculating what will happen to the packet after the pull/push operation. Which L2/L3/L4/… offsets are going to be changed and in which direction? Between which headers the application is going to add/remove layers? What header are moved? What happens next in the chain? What offsets really need an update? Application has all that knowledge and can do only those updates that are needed. We keep it simple – if application modifies the packet it is responsible to maintain metadata offsets accurate. -Petri From: ext Bill Fischofer [mailto:[email protected]<mailto:[email protected]>] Sent: Wednesday, December 17, 2014 2:06 PM To: Savolainen, Petri (NSN - FI/Espoo) Cc: Petri Savolainen; lng-odp-forward Subject: Re: [lng-odp] [PATCH] Remove packet l2/l3/l4 offset adjustments You are correct that the API states this. What others are pointing out are the non-intutive implications of choosing to define the API this way. You've not provided a convincing rationale for making this choice. While you can assert your right not to do so, it would be helpful to others if you did. On Wed, Dec 17, 2014 at 5:51 AM, Savolainen, Petri (NSN - FI/Espoo) <[email protected]<mailto:[email protected]>> wrote: The API says already: “Operation does not modify packet segmentation or move data. Handles and pointers remain valid. User is responsible to update packet meta-data offsets when needed.” If application modifies packet headers (adds/removes/changes them), it must update those offsets that it or some other block after it in the chain cares about (reloads from the packet). It may leave some offsets unchanged because it knows that those are not needed any more. Some more words can be added, if the current text is not clear enough. The current implementation does not implement the spec. This patch fixes that issue. -Petri From: ext Bill Fischofer [mailto:[email protected]<mailto:[email protected]>] Sent: Wednesday, December 17, 2014 1:33 PM To: Petri Savolainen Cc: lng-odp-forward Subject: Re: [lng-odp] [PATCH] Remove packet l2/l3/l4 offset adjustments If we do this we really need a documentation note that says: Note: After doing a head push/pull, applications cannot use routines like odp_packet_l3_ptr(), etc., until they make appropriate calls to odp_packet_l3_offset_set(), etc., to reflect the previous operation. Otherwise they will receive pointers to incorrect areas of the packet. On Wed, Dec 17, 2014 at 3:12 AM, Petri Savolainen <[email protected]<mailto:[email protected]>> wrote: Packet head push/pull calls do not automatically adjust metadata offsets, only data pointer and headroom. Signed-off-by: Petri Savolainen <[email protected]<mailto:[email protected]>> --- platform/linux-generic/include/odp_packet_internal.h | 8 -------- 1 file changed, 8 deletions(-) diff --git a/platform/linux-generic/include/odp_packet_internal.h b/platform/linux-generic/include/odp_packet_internal.h index a0eff30..18e69b3 100644 --- a/platform/linux-generic/include/odp_packet_internal.h +++ b/platform/linux-generic/include/odp_packet_internal.h @@ -192,24 +192,16 @@ static inline void *packet_map(odp_packet_hdr_t *pkt_hdr, pkt_hdr->headroom + pkt_hdr->frame_len); } -#define pull_offset(x, len) (x = x < len ? 0 : x - len) - static inline void push_head(odp_packet_hdr_t *pkt_hdr, size_t len) { pkt_hdr->headroom -= len; pkt_hdr->frame_len += len; - pkt_hdr->l2_offset += len; - pkt_hdr->l3_offset += len; - pkt_hdr->l4_offset += len; } static inline void pull_head(odp_packet_hdr_t *pkt_hdr, size_t len) { pkt_hdr->headroom += len; pkt_hdr->frame_len -= len; - pull_offset(pkt_hdr->l2_offset, len); - pull_offset(pkt_hdr->l3_offset, len); - pull_offset(pkt_hdr->l4_offset, len); } static inline void push_tail(odp_packet_hdr_t *pkt_hdr, size_t len) -- 2.2.0 _______________________________________________ lng-odp mailing list [email protected]<mailto:[email protected]> http://lists.linaro.org/mailman/listinfo/lng-odp
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