From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gandalf.ozlabs.org (mail.ozlabs.org [IPv6:2404:9400:2221:ea00::3]) by passt.top (Postfix) with ESMTPS id A48D35A0274 for ; Fri, 1 Mar 2024 00:11:27 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202312; t=1709248281; bh=PtTRymW50SYI6OqzY1AaosHe981JwQeZs0t+UjKTuec=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BUa2xX1fIhb0U5MuPQp62oy9lJs+7GNmQMRs1GNtEcrfE9+JNCKgxTtlOd6e2zw32 tWFwzN2ZxZ2pKkyG64A780Lh8GSHlSR+nuAAI1JpU/+MUp7Yvk1KbvzXZzWBleVmE2 e/ATTnxcGEvRttrigE5Kz9eiGBHXFcT+biJW+rF8l30rxLY7S7wdZA44ipR7/EVix8 8SFm6ReB4zqGQLxlyW3cFp44w0eMzaSax17qolox571wA0xnHCF09AmcSpdp5+f/vu 2lB0xLhYEO67BB+fFzEuCIAiZzeg6T3PFP8HbqhalieNR1MxbdNlkw/soGwAROiMLX EEaW9Q9DMQXCQ== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4Tm6RF4QPYz4wcY; Fri, 1 Mar 2024 10:11:21 +1100 (AEDT) Date: Fri, 1 Mar 2024 10:09:39 +1100 From: David Gibson To: Stefano Brivio Subject: Re: [PATCH v3 7/9] checksum: introduce functions to compute the header part checksum for TCP/UDP Message-ID: References: <20240217150725.661467-1-lvivier@redhat.com> <20240217150725.661467-8-lvivier@redhat.com> <04c99072-02ea-46a9-aac6-23116cb05fa1@redhat.com> <20240229080509.4f534831@elisabeth> <20240229095625.557367ab@elisabeth> <20240229151553.60d5cf18@elisabeth> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="htwTZsBupYfaRTYl" Content-Disposition: inline In-Reply-To: <20240229151553.60d5cf18@elisabeth> Message-ID-Hash: S64LUGSQM6WVFDFOWG2GC24YIH3ID6WU X-Message-ID-Hash: S64LUGSQM6WVFDFOWG2GC24YIH3ID6WU X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Laurent Vivier , passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --htwTZsBupYfaRTYl Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Feb 29, 2024 at 03:15:53PM +0100, Stefano Brivio wrote: > On Thu, 29 Feb 2024 09:56:25 +0100 > Stefano Brivio wrote: >=20 > > On Thu, 29 Feb 2024 19:49:09 +1100 > > David Gibson wrote: > >=20 > > > On Thu, Feb 29, 2024 at 08:05:09AM +0100, Stefano Brivio wrote: =20 > > > > On Thu, 29 Feb 2024 11:38:53 +1100 > > > > David Gibson wrote: > > > > =20 > > > > > On Wed, Feb 28, 2024 at 02:26:18PM +0100, Laurent Vivier wrote: = =20 > > > > > > On 2/19/24 04:08, David Gibson wrote: =20 > > > > > > > On Sat, Feb 17, 2024 at 04:07:23PM +0100, Laurent Vivier wrot= e: =20 > > > > > > > > > > > > > > [...] > > > > > > > =20 > > > > > > > > +/** > > > > > > > > + * proto_ipv6_header_psum() - Calculates the partial check= sum of an > > > > > > > > + * IPv6 header for UDP or TCP > > > > > > > > + * @payload_len: Payload length > > > > > > > > + * @proto: Protocol number > > > > > > > > + * @saddr: Source address > > > > > > > > + * @daddr: Destination address > > > > > > > > + * Returns: Partial checksum of the IPv6 header > > > > > > > > + */ > > > > > > > > +uint32_t proto_ipv6_header_psum(uint16_t payload_len, uint= 8_t protocol, > > > > > > > > + struct in6_addr saddr, struct in6_addr daddr) =20 > > > > > > >=20 > > > > > > > Hrm, this is passing 2 16-byte IPv6 addresses by value, which= might > > > > > > > not be what we want. =20 > > > > > >=20 > > > > > > The idea here is to avoid the pointer alignment problem (&ip6h-= >saddr and > > > > > > &ip6h->daddr can be misaligned). =20 > > > > >=20 > > > > > Ah, right. That's a neat idea, but I'm not sure it really helps:= I > > > > > think it will just move the misaligned access from inside the fun= ction > > > > > to the call site, where we try to marshal the parameter from some= thing > > > > > unaligned. =20 > > > >=20 > > > > I haven't tested this yet, but note that this is generally okay: the > > > > problem is *dereferencing* an unaligned pointer. But if you load me= mory > > > > from an aligned pointer, and extract a value from this memory, it's= all > > > > fine. =20 > > >=20 > > > Right, that's kind of what I'm getting at. Assuming this value starts > > > in an unaligned buffer, then in order to pass this by value the caller > > > will need to load from that unaligned pointer. AFAIK, the compiler > > > will base the type of loads only on the pointed to type, which isn't > > > changed whether we dereference in the caller or the callee. > > > =20 > > > >=20 > > > > Speaking MIPS, this is not safe on all CPU models: > > > >=20 > > > > la $1, 1002 # s1 now contains the value 1002 > > > > lw $2, 0($1) # load word from memory at 1002 + 0 into s2 > > > >=20 > > > > but this is: > > > >=20 > > > > la $1, 1000 # s1 now contains the value 1000 > > > > la $2, 1004 # s3 now contains the value 1004 > > > > lw $3, 0($1) # load word from memory at 1000 + 0 into s3 > > > > lw $4, 0($3) # load word from memory at 1004 + 0 into s4 > > > > sll $5, $3, 16 # 16-bit shift left s3 into s5 > > > > srl $6, $4, 16 # 16-bit shift right s4 into s6 > > > > or $2, $5, $6 # OR s5 and s6 into s2 =20 > > >=20 > > > Right, but I don't think merely moving the dereference to the caller > > > will necessarily induce the compiler to generate this rather than the > > > former. =20 > >=20 > > Oh, oops, I didn't realise this was the case (I haven't reviewed the > > patch yet). >=20 > ...no, that's not the case. Dereferencing 'iph' from > struct tcp[46]_l2_buf_t is fine: >=20 > struct tcp4_l2_buf_t { > uint8_t pad[2]; /* 0 2 */ > struct tap_hdr taph; /* 2 18 */ > struct iphdr iph; /* 20 20 */ > [...] > } __attribute__((__packed__)); >=20 > struct tcp6_l2_buf_t { > uint8_t pad[2]; /* 0 2 */ > struct tap_hdr taph; /* 2 18 */ > struct ipv6hdr ip6h; /* 20 40 */ > [...] > } __attribute__((__packed__)); >=20 > The problematic structures are the UDP buffers: >=20 > struct udp4_l2_buf_t { > struct sockaddr_in s_in; /* 0 16 */ > struct tap_hdr taph; /* 16 18 */ > struct iphdr iph; /* 34 20 */ > [...] > } __attribute__((__aligned__(4))); >=20 > and for UDP, this patch is dereferencing buffer pointers only, not > pointers to headers. Ok... but my point remains, I'm not seeing that passing the address by value actually helps - it just seems to change whether we need to handle the unaligned load in the caller or the callee. > Inconsistent if you want, but it's quite simple and it works, plus if you > had half a mind (at least for UDP) to split buffers into header and > payloads iovec parts... this doesn't need to be exceedingly elegant. That won't actually help this, since (for now at least) I intend to handle all the headers, including UDP, as one blob. The payload in the second iov will be the UDP payload, not the IP payload. --=20 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 --htwTZsBupYfaRTYl Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmXhDrIACgkQzQJF27ox 2GdtMQ/+PUMCYoTT+dsxQMD8EnJBKBHWuAyApa9/RqyuJgKDuWkvUO+UquuH/Ujs KX7aGAVX/nae5fYsTRGMXpx1tnX18x/1tQYcizZvdkdDuq67bG7faRa9jETkZnOo 6o1ery5XRqPrmUmcFFMmPewc0rOhMAuJYfiIIHflU5EszR5FooDNcidSnVByher0 iKJmaaf/AgHIaKcvSzLw+M+x+ec234lsKRlVi8CcMFRR/JxT6QS00LjH7WGu+VS4 08Yh6cO0fxgO8qOHryYriO55+caO4gcijZ/pUDJFKTzhLqhNlJFPKuFE0tlx4Y8s t8tgI+x/7PiNnn4kLIFCMJARTHO8VJ4mIQCcpUjuD7y09fNcCdLI3lSdxMJuGie0 oQMipCHwTQASG6TNp4/E1+5QzGAStOAi89elMbPWYIWpZEPSDADej7f3+YF6JIqa Yk9W5Cf+/RjGh/dGS3vs9kNZe/OGNEZ9WBqfGdrWUm4mH2pXVNlyFLkDnszC+cO2 H2T1j+REar9t966ZJW5p5MB7umOPvpKKnY1tVl8IKPMnrcjeCU2ufBlB2KbdDB8d bQxsO5t5lUNoZCYdZg4c97auy+hMoufr7TEKXXSwEsglL0wPhMSAVSgsDpftb4EL wUsn3RpqgnFGL+orgWRVQXGgxTPiYVBgUbMLBwS3/du7qISwvys= =4+xG -----END PGP SIGNATURE----- --htwTZsBupYfaRTYl--