Re: [PATCH 1/7] tcp: Pass TCP header and payload separately to tcp_update_check_tcp[46]()

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Oct 28, 2024 at 07:42:54PM +0100, Stefano Brivio wrote:
> On Mon, 28 Oct 2024 20:40:44 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Currently these expects both the TCP header and payload in a single IOV,
> > and goes to some trouble to locate the checksum field within it.  In the
> > current caller we've already know where the TCP header is, so we might as
> > well just pass it in.  This will need to work a bit differently for
> > vhost-user, but that code already needs to locate the TCP header for other
> > reasons, so again we can just pass it in.
> 
> We couldn't do this, and also what you're now doing in 5/7, because
> with vhost-user the TCP header is not aligned, so we can't pass it
> around as a pointer, see:
> 
>   <ZeUpxEY-sn64NLE5@zatzit>
>   https://archives.passt.top/passt-dev/ZeUpxEY-sn64NLE5@zatzit/
> 
> and following. That one is about IP headers, but the same applies to
> TCP and UDP headers.

Hrm.  I'm aware it theoretically need not be aligned, but I thought it
was in practice.. and that we were already relying on that.

In fact, I'm pretty sure the second part is true, although more subtly
than here.  v8 of the vhost-user patches calls tcp_fill_headers[46]()
with the bp parameter set to the offset of the TCP header.  If
creating a tcphdr * there is a problem, then creating a tcp_payload_t
* can't be any better.

> Of course the current solution is not elegant and it would be nice to
> find another way to deal with it, but we couldn't come up with anything
> better back then.
> 
> The rest of the series looks good to me, but I'm afraid that without
> this one and 5/7 the other changes will be a bit more complicated to
> implement (if at all possible).

Definitely.  I have so ideas for approaches more robust to
misalignment, but they're substantially more complicated.  I was
hoping we could avoid it at least for now.

-- 
David Gibson (he or they)	| 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

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