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From: Olivier Matz <olivier.matz@6wind.com>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: dev@dpdk.org, Keith Wiles <keith.wiles@intel.com>,
	Hongzhi Guo <guohongzhi1@huawei.com>,
	Thomas Monjalon <thomas@monjalon.net>
Subject: Re: [dpdk-dev] [PATCH 3/4] net: introduce functions to verify L4 checksums
Date: Wed, 28 Apr 2021 14:21:53 +0200	[thread overview]
Message-ID: <20210428122153.GU1726@platinum> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35C61718@smartserver.smartshare.dk>

Hi Morten,

Thank you for the review.

<...>

On Tue, Apr 27, 2021 at 05:07:04PM +0200, Morten Brørup wrote:
> > +static inline uint16_t
> > +rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void
> > *l4_hdr)
> > +{
> > +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> > +
> > +	cksum = ~cksum;
> > +
> >  	/*
> > -	 * Per RFC 768:If the computed checksum is zero for UDP,
> > +	 * Per RFC 768: If the computed checksum is zero for UDP,
> >  	 * it is transmitted as all ones
> >  	 * (the equivalent in one's complement arithmetic).
> >  	 */
> >  	if (cksum == 0 && ipv4_hdr->next_proto_id == IPPROTO_UDP)
> >  		cksum = 0xffff;
> > 
> > -	return (uint16_t)cksum;
> > +	return cksum;
> > +}
> 
> The GCC static branch predictor treats the above comparison as likely. Playing around with Godbolt, I came up with this alternative:
> 
> 	if (likely(cksum != 0)) return cksum;
> 	if (ipv4_hdr->next_proto_id == IPPROTO_UDP) return 0xffff;
> 	return 0;

Good idea, this is indeed an unlikely branch.
However this code was already present before this patch,
so I suggest to add it as a specific optimization patch.

> > +
> > +/**
> > + * Validate the IPv4 UDP or TCP checksum.
> > + *
> > + * @param ipv4_hdr
> > + *   The pointer to the contiguous IPv4 header.
> > + * @param l4_hdr
> > + *   The pointer to the beginning of the L4 header.
> > + * @return
> > + *   Return 0 if the checksum is correct, else -1.
> > + */
> > +__rte_experimental
> > +static inline int
> > +rte_ipv4_udptcp_cksum_verify(const struct rte_ipv4_hdr *ipv4_hdr,
> > +			     const void *l4_hdr)
> > +{
> > +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> > +
> > +	if (cksum != 0xffff)
> > +		return -1;
> 
> The GCC static branch predictor treats the above comparison as likely, so I would prefer unlikely() around it.

For this one, I'm less convinced: should we decide here whether
the good or the bad checksum is more likely than the other?

Given it's a static inline function, wouldn't it be better to let
the application call it this way:
  if (likely(rte_ipv4_udptcp_cksum_verify(...) == 0))  ?


Regards,
Olivier

  reply	other threads:[~2021-04-28 12:21 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-27 13:57 [dpdk-dev] [PATCH 0/4] net/tap: fix Rx cksum Olivier Matz
2021-04-27 13:57 ` [dpdk-dev] [PATCH 1/4] net/tap: fix Rx cksum flags on IP options packets Olivier Matz
2021-04-30 14:48   ` [dpdk-dev] [dpdk-stable] " Ferruh Yigit
2021-06-08 10:13     ` Andrew Rybchenko
2021-06-08 12:29       ` Olivier Matz
2021-06-08 12:34         ` Andrew Rybchenko
2021-06-08 12:49           ` Olivier Matz
2021-06-08 13:57             ` Andrew Rybchenko
2021-06-08 14:30               ` Olivier Matz
2021-04-27 13:57 ` [dpdk-dev] [PATCH 2/4] net/tap: fix Rx cksum flags on TCP packets Olivier Matz
2021-06-08 10:18   ` Andrew Rybchenko
2021-04-27 13:57 ` [dpdk-dev] [PATCH 3/4] net: introduce functions to verify L4 checksums Olivier Matz
2021-04-27 15:02   ` Morten Brørup
2021-04-27 15:07   ` Morten Brørup
2021-04-28 12:21     ` Olivier Matz [this message]
2021-04-28 12:42       ` Morten Brørup
2021-04-30 15:42   ` Ferruh Yigit
2021-06-08 10:23     ` Andrew Rybchenko
2021-06-08 12:29       ` Olivier Matz
2021-06-08 12:39         ` Andrew Rybchenko
2021-06-25 15:38           ` Ferruh Yigit
2021-04-27 13:57 ` [dpdk-dev] [PATCH 4/4] test/cksum: new test for L3/L4 checksum API Olivier Matz
2021-06-30 13:51 ` [dpdk-dev] [PATCH v2 0/4] net/tap: fix Rx cksum Olivier Matz
2021-06-30 13:51   ` [dpdk-dev] [PATCH v2 1/4] net/tap: fix Rx cksum flags on IP options packets Olivier Matz
2021-06-30 13:51   ` [dpdk-dev] [PATCH v2 2/4] net/tap: fix Rx cksum flags on TCP packets Olivier Matz
2021-06-30 13:51   ` [dpdk-dev] [PATCH v2 3/4] net: introduce functions to verify L4 checksums Olivier Matz
2021-06-30 13:51   ` [dpdk-dev] [PATCH v2 4/4] test/cksum: new test for L3/L4 checksum API Olivier Matz
2021-07-01  9:28   ` [dpdk-dev] [PATCH v2 0/4] net/tap: fix Rx cksum Andrew Rybchenko

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