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Message-ID: <20190429104414.GB17493@osadl.at> Date: Mon, 29 Apr 2019 12:44:14 +0200 From: Nicholas Mc Guire <der.herr@...r.at> To: Edward Cree <ecree@...arflare.com> Cc: Nicholas Mc Guire <hofrat@...dl.org>, Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] net_sched: force endianness annotation On Mon, Apr 29, 2019 at 11:11:20AM +0100, Edward Cree wrote: > On 28/04/2019 06:54, Nicholas Mc Guire wrote: > > While the endiannes is being handled correctly sparse was unhappy with > > the missing annotation as be16_to_cpu()/be32_to_cpu() expects a __be16 > > respectively __be32. > [...] > > diff --git a/net/sched/em_cmp.c b/net/sched/em_cmp.c > > index 1c8360a..3045ee1 100644 > > --- a/net/sched/em_cmp.c > > +++ b/net/sched/em_cmp.c > > @@ -41,7 +41,7 @@ static int em_cmp_match(struct sk_buff *skb, struct tcf_ematch *em, > > val = get_unaligned_be16(ptr); > > > > if (cmp_needs_transformation(cmp)) > > - val = be16_to_cpu(val); > > + val = be16_to_cpu((__force __be16)val); > > break; > There should probably be a comment here to explain what's going on. TBH > it's probably a good general rule that any use of __force should have a > comment explaining why it's needed. > AFAICT, get_unaligned_be16(ptr) is (barring alignment) equivalent to > be16_to_cpu(*(__be16 *)ptr). But then calling be16_to_cpu() again on > val is bogus; it's already CPU endian. There's a distinct lack of > documentation around as to the intended semantics of TCF_EM_CMP_TRANS, > but it would seem either (__force u16)cpu_to_be16(val); (which preserves > the existing semantics, that trans is a no-op on BE) or swab16(val); > would make more sense. > be16_to_cpu((__force __be16)val) should be a NOP on big-endian as well - atleast that is how I understood it (usr/include/linux/byteorder/big_endian.h). The problem with using swab16 is that it is impating the binary significantly so I'm not sure if the change is really side-effect free - while the somewhat brute force solution is evaluatable simply by diffing. The swab16() solution seems cleaner than adding another layer of casting - but I just am unsure if - val = be16_to_cpu(val); + val = swab16(val); is actually equivalent. For the original patch this can be checked -rw-r--r-- 1 hofrat hofrat 2984 Apr 28 01:49 /tmp/em_cmp_force.o -rw-r--r-- 1 hofrat hofrat 2984 Apr 28 01:49 /tmp/em_cmp_org.o -rw-r--r-- 1 hofrat hofrat 3392 Apr 29 06:25 /tmp/em_cmp_swab.o hofrat@...ian:~/linux-next$ diff /tmp/em_cmp_force.o /tmp/em_cmp_org.o hofrat@...ian:~/linux-next$ which is why I prefered that solution. if swab16() is equivalent I' resend a V2 thx! hofrat
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