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:
 <SA3PR21MB3867B36A9565AB01B0114D3ACA8EA@SA3PR21MB3867.namprd21.prod.outlook.com>
Date: Tue, 13 Jan 2026 15:09:15 +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: Monday, January 12, 2026 8:22 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 Mon, 12 Jan 2026 21:01:59 +0000 Haiyang Zhang wrote:
> > > > Our NIC can have up to 4 RX packets on 1 CQE. To support this
> feature,
> > > > check and process the type CQE_RX_COALESCED_4. The default setting
> is
> > > > disabled, to avoid possible regression on latency.
> > > >
> > > > And add ethtool handler to switch this feature. To turn it on, run:
> > > >   ethtool -C <nic> rx-frames 4
> > > > To turn it off:
> > > >   ethtool -C <nic> rx-frames 1
> > >
> > > Exposing just rx frame count, and only two values is quite unusual.
> > > Please explain in more detail the coalescing logic of the device.
> > Our NIC device only supports coalescing on RX. And when it's disabled
> each
> > RX CQE indicates 1 RX packet; when enabled each RX CQE indicates up to 4
> packets.
> 
> 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.

Thanks,
- Haiyang