Re: [PATCH] ip: Use regular htons() for non-constant protocol number in L2_BUF_IP4_PSUM

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Mar 08, 2024 at 10:28:38AM +0100, Stefano Brivio wrote:
> On Fri, 8 Mar 2024 11:55:32 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Wed, Mar 06, 2024 at 08:08:36AM +0100, Stefano Brivio wrote:
> > > instead of htons_constant(), which is for... constants.
> > > 
> > > Fixes: 5bf200ae8a1a ("tcp, udp: Don't include destination address in partially precomputed csums")
> > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> > 
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> > 
> > It seems to get the job done, at what's probably negligible practical
> > cost.  My perfectionist side has some misgivings:
> > 
> > AIUI, the point of htons_constant() isn't so much that it *requires* a
> > constant, but that because it's open-coded in plain arithmetic
> > operations the compiler will be able to constant fold it, if it is
> > invoked with a constant parameter.
> 
> Right, yes, it doesn't require a constant. Still, I'd argue it's meant
> for constants. :)
> 
> > Since htons() is a library
> > function, it probably can't be elided in that way.  The cost of
> > htons_constant() is that it may be a less optimal implementation when
> > the calculation really does need to be done at runtime.
> > 
> > I'm still a bit baffled at that Coverity warning: I can't see why it
> > doesn't preclude any situation where you want to write out
> > calculations for clarity, even though you know the result will be a
> > constant (and you expect the compiler to fold it).
> 
> ...maybe it actually does preclude any situation like that? This is the
> only example we have with __bswap_constant16(), and variables mixed
> (ORed) with constants.

Right, but AFAICT there's nothing that should be specific to
bswap_constant here, since that's just expanding to some shifts and
masks.

> Other usages of __bswap_constant16() have just a variable as argument
> (no "problem" with that, of course), and we use __bswap_constant_32()
> with constants only.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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