Re: [dpdk-dev] RFC - adding filter to packet capture API

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> In the process of updating packet capture to take a filter program, there
> is one open question about API/ABI.
> 
> The example is:
> 
> int
> rte_pdump_enable(uint16_t port, uint16_t queue, uint32_t flags,
> 		struct rte_ring *ring,
> 		struct rte_mempool *mp,
> 		void *filter);
> 
> For the new version want to add ability to pass a BPF (classic) program
> from libcap. This is described in most pcap API's as "struct bpf_program".
> 
> The filter parameter was left as a placeholder, but in typical YAGNI
> fashion. When we do need to use it, it is not going to work out.
> 
> Since the existing filter argument was never used, there are a bunch
> of options (in order from best to worse).
> 
> 1. Introduce new API which takes a filter.
> 
> int
> rte_pdump_enable_bpf(uint16_t port, uint16_t queue, uint32_t flags,
> 		struct rte_ring *ring,
> 		struct rte_mempool *mp,
> 		const struct bpf_program *filter);
> 
> The old API is just the same as calling new API with NULL as filter.
> This solution is safe but adds minor bloat.
> 
> 2. Use API versioning.  This solves the ABI problem but it is still
>    an API breakage since program that was passing junk would still
>    not compile.
> 
> 3. Change the function signature of existing API. This risks breaking
>    at compile time some program that was passing some other value.
>    Similarly, a program could have passed garbage, we never checked.
> 
> 4. Keep existing function signature, but be type unsafe.
>    This keeps API, but still is ABI breakage for programs that passed
>    garbage. Plus C is unsafe enough already.
> 

My preference is probably #4, with some extra changes:
make actual type for 'filter' be determined by flags,
something like:

enum {
        RTE_PDUMP_FLAG_RX = 1,  /* receive direction */
        RTE_PDUMP_FLAG_TX = 2,  /* transmit direction */
+      RTE_PDUMP_FLAG_CBPF = 4, /* filter points to struct bpf_program */
        /* both receive and transmit directions */
        RTE_PDUMP_FLAG_RXTX = (RTE_PDUMP_FLAG_RX|RTE_PDUMP_FLAG_TX)
};