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

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Message-ID: <Zs8Tb2HXO7b9BbYn@shredder.mtl.com>
Date: Wed, 28 Aug 2024 15:09:19 +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 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?

> It's implemented in the patches I previously shared:
> 
> https://github.com/idosch/linux/commit/a3289a6838a0d0e6e0a30a61132bdce3d2f71a3c.patch
> https://github.com/idosch/linux/commit/ff5dd634fb278431b58437654d7f65b57fd4ae4b.patch
> https://github.com/idosch/linux/commit/3060ecb534475eadabfa1d419dd64804f0bd0148.patch
> https://github.com/idosch/linux/commit/12ddbce4f519b42477ea1e130b6d2bab1cca137c.patch
> 
> > 
> > > need to make sure the entire DSCP value is present in the IPv4 flow key.
> > > This patchset continues to unmask the upper DSCP bits, but this time in
> > > the output route path.
> >