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

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 29 Oct 2024 15:07:56 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Oct 29, 2024 at 02:02:25PM +1100, David Gibson wrote:
> > 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.

Ah, okay, I missed that. Still, I think we should ask gcc for an
opinion (with the vhost-user series on top of this series), because
those build-time pointer alignment checks are pretty reliable.

> > > 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.  
> 
> I had a closer look at that earlier message now.  I believe at the
> time I was aiming for fully robust handling of misaligned user
> buffers.  AIUI, we've given up on that for the time being: instead
> we'll just *test* for suitable alignment and we can do the hard work
> of handling it if it ever arises in practice.

Right, and we can use the compiler to test for suitable alignment.

-- 
Stefano