linux-kernel - Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support for coalesced RX packets on CQE

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Message-ID: <20260113170948.1d6fbdaf@kernel.org>
Date: Tue, 13 Jan 2026 17:09:48 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...ux.microsoft.com>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, KY Srinivasan
 <kys@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
 <DECUI@...rosoft.com>, Long Li <longli@...rosoft.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Konstantin
 Taranov <kotaranov@...rosoft.com>, Simon Horman <horms@...nel.org>, Erni
 Sri Satya Vennela <ernis@...ux.microsoft.com>, Shradha Gupta
 <shradhagupta@...ux.microsoft.com>, Saurabh Sengar
 <ssengar@...ux.microsoft.com>, Aditya Garg
 <gargaditya@...ux.microsoft.com>, Dipayaan Roy
 <dipayanroy@...ux.microsoft.com>, Shiraz Saleem
 <shirazsaleem@...rosoft.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-rdma@...r.kernel.org"
 <linux-rdma@...r.kernel.org>, Paul Rosswurm <paulros@...rosoft.com>
Subject: Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support
 for coalesced RX packets on CQE

On Tue, 13 Jan 2026 15:13:24 +0000 Haiyang Zhang wrote:
> > > I get that. What is the logic for combining 4 packets into a single
> > > completion? How does it work? Your commit message mentions "regression
> > > on latency" - what is the bound on that regression?  
> > 
> > When we received CQE type CQE_RX_COALESCED_4, it's a coalesced CQE. And in
> > the CQE OOB, there is an array with 4 PPI elements, with each pkt's length:
> > oob->ppi[i].pkt_len.
> > 
> > So we read the related WQE and the DMA buffers for the RX pkt payloads, up
> > to 4.
> > But, if the coalesced pkts <4, the pkt_len will be 0 after the last pkt,
> > so we know when to stop reading the WQEs.  
> 
> And, the coalescing can add up to 2 microseconds into one-way latency.

I am asking you how the _device_ (hypervisor?) decides when to coalesce
and when to send a partial CQE (<4 packets in 4 pkt CQE). You are using
the coalescing uAPI, so I'm trying to make sure this is the correct API.
CQE configuration can also be done via ringparam.