Re: [PATCH 3/7] tcp_conn: Avoid 7-bit hole in struct tcp_splice_conn

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

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On Thu, Jan 30, 2025 at 05:55:00AM +0100, Stefano Brivio wrote:
> On Thu, 30 Jan 2025 11:44:19 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Wed, Jan 29, 2025 at 08:33:40AM +0100, Stefano Brivio wrote:
> > > On Wed, 29 Jan 2025 12:02:09 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Tue, Jan 28, 2025 at 07:48:33AM +0100, Stefano Brivio wrote:  
> > > > > On Tue, 28 Jan 2025 11:53:09 +1100
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >     
> > > > > > On Tue, Jan 28, 2025 at 12:15:28AM +0100, Stefano Brivio wrote:    
> > > > > > > Moving in_epoll out of the common flow data created a 7-bit hole in
> > > > > > > struct tcp_splice_conn: repack by shrinking @flags by one (otherwise
> > > > > > > unused) bit.      
> > > > > > 
> > > > > > Is this actually necessary for the migration stuff?  Or just a cleanup
> > > > > > you spotted along the way?    
> > > > > 
> > > > > I thought it was helpful to keep the same size on 32-bit, but it looks
> > > > > like it's not actually needed.
> > > > > 
> > > > > Let me drop it from this series as it's just noise and I'm trying to
> > > > > keep this slim. If we are all happy with it I can apply it. If not I'll
> > > > > forget about it.    
> > > > 
> > > > Eh, I don't care that much either way.
> > > > 
> > > > Note, btw, that bit-field packing is another way source and
> > > > destination could potentially have mismatching data structures.  IIUC
> > > > bit field packing is described by the ABI and doesn't necessarily
> > > > match the byte endianness.  
> > > 
> > > Right, that's actually the reason that brought me to this change: I was
> > > comparing stuff between x86_64 and armv6l. On the other hand, this part
> > > of the specific ABI is generally considered stable so I can rely on it.  
> > 
> > Uhh.. a specific ABI is stable, yes, but IIUC the whole point of these
> > endian, word size etc. checks is that you're not counting on it being
> > an identical ABI at each end.
> 
> Of course. I'm just saying that I can *rely on ABIs*. Not on them being
> the same.
> 
> > I'm saying the bit field packing is another way the ABIs at each end
> > could differ
> 
> It does.
> 
> > which is not currently accounted for.
> 
> That's because I have two hands, but obviously if I'm comparing ABIs...

Ok, fair enough.

-- 
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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