[xdp-hints] Re: [RFC bpf-next 01/23] ice: make RX hash reading code more reusable

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Thu, Sep 14, 2023 at 06:12:23PM +0200, Alexander Lobakin wrote:
> From: Larysa Zaremba <larysa.zaremba@intel.com>
> Date: Thu, 24 Aug 2023 21:26:40 +0200
> 
> > Previously, we only needed RX hash in skb path,
> > hence all related code was written with skb in mind.
> > But with the addition of XDP hints via kfuncs to the ice driver,
> > the same logic will be needed in .xmo_() callbacks.
> > 
> > Separate generic process of reading RX hash from a descriptor
> > into a separate function.
> > 
> > Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> 
> I like the patch, except three minors above,
> 
> > ---
> >  drivers/net/ethernet/intel/ice/ice_txrx_lib.c | 37 +++++++++++++------
> >  1 file changed, 26 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> > index c8322fb6f2b3..8f7f6d78f7bf 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> > @@ -63,28 +63,43 @@ static enum pkt_hash_types ice_ptype_to_htype(u16 ptype)
> >  }
> >  
> >  /**
> > - * ice_rx_hash - set the hash value in the skb
> > + * ice_get_rx_hash - get RX hash value from descriptor
> > + * @rx_desc: specific descriptor
> > + *
> > + * Returns hash, if present, 0 otherwise.
> > + */
> > +static u32
> > +ice_get_rx_hash(const union ice_32b_rx_flex_desc *rx_desc)
> 
> The whole declaration could easily fit into one line :>
>

I agree
 
> > +{
> > +	const struct ice_32b_rx_flex_desc_nic *nic_mdid;
> > +
> > +	if (rx_desc->wb.rxdid != ICE_RXDID_FLEX_NIC)
> 
> Not really related: have you tried to measure branch hit/miss here?
> Can't it be a candidate for unlikely()?

I have not measured this, but at least in my test setup, I have never seen any 
other rxdid, so unlikely() is a good idea. If it harms some particular 
applications, we can always remove this later on request :D

> > +		return 0;
> > +
> > +	nic_mdid = (struct ice_32b_rx_flex_desc_nic *)rx_desc;
> > +	return le32_to_cpu(nic_mdid->rss_hash);
> 
> I think the common convention in the kernel is to separate the last
> return from the main body with a newline.
> To not leave the cast above alone, you can embed it into the declaration.
> 

I am fine with leaving the cast alone.

> 	const struct ice_32b_rx_flex_desc_nic *mdid = (typeof(mdid))rx_desc;
> 
> This is a compile-time cast w/o any maths anyway, so doing it before
> checking for the descriptor type doesn't hurt in any way.
> 
> 	if (!= FLEX)
> 		return 0;
> 
> 	return le32_ ...
> 
> (or via a ternary)
> 
> > +}
> 
> [...]
> 
> Thanks,
> Olek