netdev - 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:
 <SA3PR21MB38676C98AA702F212CE391E2CA8FA@SA3PR21MB3867.namprd21.prod.outlook.com>
Date: Wed, 14 Jan 2026 18:27:50 +0000
From: Haiyang Zhang <haiyangz@...rosoft.com>
To: Jakub Kicinski <kuba@...nel.org>
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



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Tuesday, January 13, 2026 8:10 PM
> To: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...ux.microsoft.com>; linux-
> hyperv@...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-
> 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.

When coalescing is enabled, the device waits for packets which can 
have the CQE coalesced with previous packet(s). That coalescing process 
is finished (and a CQE written to the appropriate CQ) when the CQE is 
filled with 4 pkts, or time expired, or other device specific logic is 
satisfied.

Thanks,
- Haiyang