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

[Stefano Brivio <sbrivio@redhat.com>]

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.

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.

-- 
Stefano