Re: [PATCH v3 7/9] checksum: introduce functions to compute the header part checksum for TCP/UDP

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 29 Feb 2024 11:38:53 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Feb 28, 2024 at 02:26:18PM +0100, Laurent Vivier wrote:
> > On 2/19/24 04:08, David Gibson wrote:  
> > > On Sat, Feb 17, 2024 at 04:07:23PM +0100, Laurent Vivier wrote:  
> > >
> > > [...]
> > >
> > > > +/**
> > > > + * proto_ipv6_header_psum() - Calculates the partial checksum 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, uint8_t protocol,
> > > > +				struct in6_addr saddr, struct in6_addr daddr)  
> > > 
> > > Hrm, this is passing 2 16-byte IPv6 addresses by value, which might
> > > not be what we want.  
> > 
> > The idea here is to avoid the pointer alignment problem (&ip6h->saddr and
> > &ip6h->daddr can be misaligned).  
> 
> 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 function
> to the call site, where we try to marshal the parameter from something
> unaligned.

I haven't tested this yet, but note that this is generally okay: the
problem is *dereferencing* an unaligned pointer. But if you load memory
from an aligned pointer, and extract a value from this memory, it's all
fine.

Speaking MIPS, this is not safe on all CPU models:

	la	$1, 1002   # s1 now contains the value 1002
	lw	$2, 0($1)  # load word from memory at 1002 + 0 into s2

but this is:

	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

On x86, as far as I know, mov will digest the equivalent of the first
version without issues.

> > Is it a better solution to copy the content of ip6h->saddr and ip6h->daddr
> > to some local variables and then provide the pointers of the local variables
> > to proto_ipv6_header_psum()?  
> 
> Honestly, I'm not sure.

I think it's pretty much the same. Let the compiler pass 16-byte
variables by value, and it will generally do this for us, but only if
needed.

-- 
Stefano