Re: [dpdk-dev] [PATCH] config: default to shared library

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Mar 04, 2015 at 03:24:12PM +0200, Panu Matilainen wrote:
> On 03/04/2015 03:08 PM, Bruce Richardson wrote:
> >On Wed, Mar 04, 2015 at 06:28:05AM -0500, Neil Horman wrote:
> >>On Wed, Mar 04, 2015 at 01:05:07PM +0200, Panu Matilainen wrote:
> >>>On 03/04/2015 11:24 AM, Thomas Monjalon wrote:
> >>>>Hi Panu,
> >>>>
> >>>>2015-03-04 08:17, Panu Matilainen:
> >>>>>With symbol versioning its vital that developers test their code in
> >>>>>shared library mode, otherwise we'll be playing "add the forgotten
> >>>>>symbol export" from here to eternity.
> >>>>
> >>>>Yes we must improve the sanity checks.
> >>>>A lot of options must be tested (or removed) and not only shared libs.
> >>>>But the error you reported before (missing export of rte_eth_dev_release_port)
> >>>>cannot be seen even with this patch.
> >>>
> >>>I know, I didn't say it would have directly caught it. It would've likely
> >>>been found earlier though, if nothing else then in testing of the new
> >>>librte_pmd_null which clearly nobody had tried in shared lib configuration.
> >>>
> >>This is accurate.  The default config is a tool, in the sense that it leverages
> >>the implicit testing of any users who are experimenting with the DPDK.  Any
> >>users out there using the DPDK test/example applications would have realized
> >>something was amiss when the testpmd app refused to run with the null or pcap
> >>pmd, since there was a missing symbol.  That "social fuzzing" has value, but it
> >>only works if the defaults are carefully selected.  Currently, building for
> >>shared libraries exposes more existing bugs than static libraries, and so we
> >>should set that as our default so as to catch them.
> >>
> >>>>It means we need more tools.
> >>>>Though, default configuration is not a tool.
> >>>
> >>>Yes, default config is not a tool, its a recommendation of sorts both for
> >>>developers and users. It also tends to be the setup that is rarely broken
> >>>because it happens to get the most testing :)
> >>>
> >>And it is a tool (see above).
> >>
> >>>>
> >>>>>By defaulting to shared we should catch more of these cases early,
> >>>>>but without taking away anybodys ability to build static.
> >>>>
> >>>>Shared libraries are convenient for distributions but have a performance
> >>>>impact. I think that static build must remain the default choice.
> >>>
> >>
> >>If utmost performance is the concern, isn't it reasonable to assume that users
> >>in that demographic will customize their configuration to achieve that?  No one
> >>assumes that something is tuned to be perfect for their needs out of the box if
> >>their needs are extreemely biased to a single quality.  The best course of
> >>action here is to set the default to be adventageous toward catching bugs, and
> >>document the changes needed to bias for performance.
> >>
> >>>For distros, this is not a matter of *convenience*, its the only technically
> >>>feasible choice.
> >
> >As I understand it, build for the "default" cpu rather than "native" is the only
> >feasible choice also, so how about re-introducing a new defconfig file for
> >"default" (or perhaps better name), where you have lowest-common denominator
> >instruction-set and building for shared libraries?
> >Would that work for everyone, or do people feel it would be too confusing to have
> >more defconfig files available?
> 
> Given the opposition to defaulting to shared, another config file seems like
> a fair compromise to me, whether "default" or something else. As for the
> naming, one possibility would be calling it "shared", implying both
> lowest-common denominator instruction set to be shareable across many
> systems and shared libraries.
> 
> 	- Panu -

The naming scheme for configs is meant to be:
"ARCH-MACHINE-EXECENV-TOOLCHAIN" 
as documented in the Getting Started Guide. "Default" has been used up till now
to refer to the lowest common denominator instruction set supported, which for
x86_64 is a core2 baseline, I believe. "shared" doesn't really fit into this
naming scheme, and there is nothing to allow extra notes to be added to the
name.
Without changing this scheme, I would suggest we rename "default" to "generic",
which I think is a slightly better term for it, and we set the
"x86_64-generic-linuxapp-gcc" target to build shared libs.

/Bruce