netdev - Re: [EXTERNAL] Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib driver.

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Date: Wed, 2 Aug 2023 04:11:18 +0000
From: Ajay Sharma <sharmaajay@...rosoft.com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: Long Li <longli@...rosoft.com>, Wei Hu <weh@...rosoft.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>, "leon@...nel.org"
	<leon@...nel.org>, KY Srinivasan <kys@...rosoft.com>, Haiyang Zhang
	<haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>, Dexuan
 Cui <decui@...rosoft.com>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, vkuznets
	<vkuznets@...hat.com>, "ssengar@...ux.microsoft.com"
	<ssengar@...ux.microsoft.com>, "shradhagupta@...ux.microsoft.com"
	<shradhagupta@...ux.microsoft.com>
Subject: Re: [EXTERNAL] Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt
 support to mana ib driver.



> On Aug 1, 2023, at 6:46 PM, Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> On Tue, Aug 01, 2023 at 07:06:57PM +0000, Long Li wrote:
> 
>> The driver interrupt code limits the CPU processing time of each EQ
>> by reading a small batch of EQEs in this interrupt. It guarantees
>> all the EQs are checked on this CPU, and limits the interrupt
>> processing time for any given EQ. In this way, a bad EQ (which is
>> stormed by a bad user doing unreasonable re-arming on the CQ) can't
>> storm other EQs on this CPU.
> 
> Of course it can, the bad use just creates a million EQs and pushes a
> bit of work through them constantly. How is that really any different
> from pushing more EQEs into a single EQ?
> 
> And how does your EQ multiplexing work anyhow? Do you poll every EQ on
> every interrupt? That itself is a DOS vector.
> 
> Jason

User does not create eqs directly . EQ creation is by product of opening device ie allocating context. I am not sure if the same process is allowed to open device multiple times - must be some kind of lock implemented. So million eqs are probably far fetched .
As for how the eq servicing is done - only those eq’s for which the interrupt is raised are checked. And each eq is tied only once and only to a single interrupt. 

Ajay