netdev - Re: [PATCH net-next 00/12] Unmask upper DSCP bits

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Message-ID: <ZtCLFYdbw6rPinwS@shredder.mtl.com>
Date: Thu, 29 Aug 2024 17:52:05 +0300
From: Ido Schimmel <idosch@...dia.com>
To: Guillaume Nault <gnault@...hat.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
	pabeni@...hat.com, edumazet@...gle.com, dsahern@...nel.org,
	ast@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev,
	john.fastabend@...il.com, steffen.klassert@...unet.com,
	herbert@...dor.apana.org.au, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 00/12] Unmask upper DSCP bits - part 2

On Thu, Aug 29, 2024 at 01:54:46PM +0200, Guillaume Nault wrote:
> On Wed, Aug 28, 2024 at 03:09:19PM +0300, Ido Schimmel wrote:
> > On Tue, Aug 27, 2024 at 06:45:53PM +0300, Ido Schimmel wrote:
> > > On Tue, Aug 27, 2024 at 03:47:05PM +0200, Guillaume Nault wrote:
> > > > On Tue, Aug 27, 2024 at 02:18:01PM +0300, Ido Schimmel wrote:
> > > > > tl;dr - This patchset continues to unmask the upper DSCP bits in the
> > > > > IPv4 flow key in preparation for allowing IPv4 FIB rules to match on
> > > > > DSCP. No functional changes are expected. Part 1 was merged in commit
> > > > > ("Merge branch 'unmask-upper-dscp-bits-part-1'").
> > > > > 
> > > > > The TOS field in the IPv4 flow key ('flowi4_tos') is used during FIB
> > > > > lookup to match against the TOS selector in FIB rules and routes.
> > > > > 
> > > > > It is currently impossible for user space to configure FIB rules that
> > > > > match on the DSCP value as the upper DSCP bits are either masked in the
> > > > > various call sites that initialize the IPv4 flow key or along the path
> > > > > to the FIB core.
> > > > > 
> > > > > In preparation for adding a DSCP selector to IPv4 and IPv6 FIB rules, we
> > > > 
> > > > Hum, do you plan to add a DSCP selector for IPv6? That shouldn't be
> > > > necessary as IPv6 already takes all the DSCP bits into account. Also we
> > > > don't need to keep any compatibility with the legacy TOS interpretation,
> > > > as it has never been defined nor used in IPv6.
> > > 
> > > Yes. I want to add the DSCP selector for both families so that user
> > > space would not need to use different selectors for different families.
> > 
> > Another approach could be to add a mask to the existing tos/dsfield. For
> > example:
> > 
> > # ip -4 rule add dsfield 0x04/0xfc table 100
> > # ip -6 rule add dsfield 0xf8/0xfc table 100
> > 
> > The default IPv4 mask (when user doesn't specify one) would be 0x1c and
> > the default IPv6 mask would be 0xfc.
> > 
> > WDYT?
> 
> For internal implementation, I find the mask option elegant (to avoid
> conditionals). But I don't really like the idea of letting user space
> provide its own mask. This would let the user create non-standard
> behaviours, likely by mistake (as nobody seem to ever have requested
> that flexibility).
> 
> I think my favourite approach would be to have the new FRA_DSCP
> attribute work identically on both IPv4 and IPv6 FIB rules and keep
> the behaviour of the old "tos" field of struct fib_rule_hdr unchanged.
> 
> This "tos" field would still work differently for IPv4 and IPv6, as it
> always did, but people wanting consistent behaviour could just use
> FRA_DSCP instead. Also, FRA_DSCP accepts real DSCP values as defined in
> RFCs, while "tos" requires the 2 bits shift. For all these reasons, I'm
> tempted to just consider "tos" as a legacy option used only for
> backward compatibility, while FRA_DSCP would be the "clean" interface.
> 
> Is that approach acceptable for you?

Yes. The patches I shared implement this approach :)