From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH RFC 0/3] net: Add driver helper function to determine if checksum is offloadable Date: Tue, 06 Oct 2015 11:51:35 +0100 Message-ID: <1444128695.4674.226.camel@infradead.org> References: <1444088364-2839440-1-git-send-email-tom@herbertland.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-BsYDdZsADGrxltzd3VxK" Cc: kernel-team@fb.com, ogerlitz@mellanox.com To: Tom Herbert , davem@davemloft.net, netdev@vger.kernel.org Return-path: Received: from bombadil.infradead.org ([198.137.202.9]:52516 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751451AbbJFKvj (ORCPT ); Tue, 6 Oct 2015 06:51:39 -0400 In-Reply-To: <1444088364-2839440-1-git-send-email-tom@herbertland.com> Sender: netdev-owner@vger.kernel.org List-ID: --=-BsYDdZsADGrxltzd3VxK Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2015-10-05 at 16:39 -0700, Tom Herbert wrote: > When drivers have support for offloading transport IP checksums, they > will indicate this in the features for the device. As described in > skbuff.h, a driver will usually advertise NETIF_F_HW_CSUM, > NETIF_F_IP_CSUM, and/or NETIF_F_IPV6_CSUM. The first of these > (NETIF_F_HW_CSUM) is the preferred method since this implies that the > device has implemented a generic checksum offload feature that should > work under arbitrary scenarios (e.g. for different protocols, with or > without encapsulation). >=20 > For narrow features support (NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM > for offload of TCP/UDP), the features flags may not be sufficient to > deduce whether a packet may be offloaded. Some devices will not be able > to offload encapsulated checksums, some cannot offload transport > checksums in packet with IPv6 extension headers, etc. In these cases > a driver will need to perform additional packet inspection to determine > if a packet's checksum can be offloaded to a device. >=20 > This patch defines a helper function that drivers can call to check if > it is able to offload the checksum for a particular packet. In an > argument to the function, the driver specifies what type of packets it > is able to offload to a device. The function is intended to check for > the most common restrictions of devices (like by IP version, transport > protocol, encapsulation, extension headers). Since the function includes > checks for IP version and transport protocol, the driver is able > to advertise NETIF_F_HW_CSUM instead of protocol specific support. > This should put us on a path to deprecate NETIF_F_IP_CSUM and > NETIF_F_IPV6_CSUM. >=20 > The helper function may be called from ndo_features_check or the xmit > routine of the driver. The csum_help argument does not need to be set > when the function is called from ndo_features_check. Looks good in general; thanks. I do strongly believe we want to encourage your helper to be called from .ndo_features_check(). Because if you can't do the checksum, then you can't do TSO. And if you can't do TSO, you *really* want your hard_start_xmit() function to be handed one skb at a time and not have to call skb_gso_segment() for itself when it might not have enough room in its descriptor ring for *all* the resulting segments. Also, is your skb_csum_offload_chk() going to do the right thing for SCTP packets? Looks like it doesn't do crc32... And how accurate is the check we have, on the various IP output routines, for rt->dst.dev->features & NETIF_F_xx_CSUM? Could we consider just ditching that check and always using CHECKSUM_PARTIAL, in the knowledge that we check it before it hits a non-capable driver anyway? Or do we benefit from doing the software checksum early, due to improved cache locality when we've probably just copied the data from userspace in many cases? --=20 David Woodhouse Open Source Technology Centre David.Woodhouse@intel.com Intel Corporation --=-BsYDdZsADGrxltzd3VxK Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIISjjCCBicw ggUPoAMCAQICAw3vNzANBgkqhkiG9w0BAQUFADCBjDELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0 YXJ0Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcx ODA2BgNVBAMTL1N0YXJ0Q29tIENsYXNzIDEgUHJpbWFyeSBJbnRlcm1lZGlhdGUgQ2xpZW50IENB MB4XDTE1MDUwNTA5NDM0MVoXDTE2MDUwNTA5NTMzNlowQjEcMBoGA1UEAwwTZHdtdzJAaW5mcmFk ZWFkLm9yZzEiMCAGCSqGSIb3DQEJARYTZHdtdzJAaW5mcmFkZWFkLm9yZzCCASIwDQYJKoZIhvcN AQEBBQADggEPADCCAQoCggEBAMkbm9kPbx1j/X4RVyf/pPKSYwelcco69TvnQQbKM8m8xkWjXJI1 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