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 19:49:09 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Feb 29, 2024 at 08:05:09AM +0100, Stefano Brivio wrote:
> > 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.  
> 
> 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.
> 
> > 
> > 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  
> 
> 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.

Oh, oops, I didn't realise this was the case (I haven't reviewed the
patch yet).

-- 
Stefano