[ovs-dev] [PATCH v1] configure: Allow opt-in to CPU ISA opts at compile time

[Van Haaren, Harry]

> -----Original Message-----
> From: Eelco Chaudron <echaudro at redhat.com>
> Sent: Wednesday, September 8, 2021 9:16 AM
> To: Amber, Kumar <kumar.amber at intel.com>
> Cc: ovs-dev at openvswitch.org; ktraynor at redhat.com; i.maximets at ovn.org;
> Stokes, Ian <ian.stokes at intel.com>; fbl at sysclose.org; Van Haaren, Harry
> <harry.van.haaren at intel.com>
> Subject: Re: [PATCH v1] configure: Allow opt-in to CPU ISA opts at compile time
> 
> Not a real review of the patch, but just some comment/questions glancing over
> the patch.

Sure, thanks for input.

> On 3 Sep 2021, at 15:53, Kumar Amber wrote:
> 
> > This commit allows "opt-in" to CPU ISA optimized implementations of
> > OVS SW datapath components at compile time. This can be useful in some
> > deployments where the CPU ISA optimized implementation is to be chosen
> > by default.

<snip some contents>

> > +Enabling all AVX512 options
> > +---------------------------
> > +
> > +A user can enable all the three DPIF, Miniflow Extract and DPLCS optimized
> > +AVX512 options at build time, if the CPU supports the required AVX512 ISA
> > +by using the following command ::
> > +
> > +    ./configure --enable-cpu-isa
> 
> If we have different ISA architectures, i.e., i86 vs ARM, we are ok with a single
> option. Have you thought about AMD adding its own AMDXXX instructions in
> addition to AVX512? How would this configuration option work? Maybe an
> optional option to prioritize one over the other.

The ISA enabling efforts have been generic so far, any reference to specific ISA (e.g. AVX512)
has been solely in the implementation choice - never in a general component. Intention here
is to stay in line with that - and "enable CPU ISA" seemed a logical string to achieve that to me..

It is of course possible to provide multiple configure command lines, but I was hoping to avoid
creating too many compile time flags. Typically I think projects attempt to avoid due to expanding
testing & validation. A single flag would limit overhead to the minimum...

Typically ISA sets have a "good - better - best" type relationship - which could lead to a general
acceptance of what ISA is best. We have runtime functions to switch implementation - so today
the code already enables a log of runtime/dynamic updating of implementation. If there's a
need to expose that at compile time too, then that's easy to add - but comes with a burden in
testing & validation...


> Even if we decide a single option is right, this one to me looks more about
> compile-time flags than enable ISA-specific code. Maybe something more in line
> with --prioritize-cpu-isa-functions? Or some other catchy name.

Sure - that string is fine for me too.

<snip patch contents>

Regards, -Harry