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

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Date: Fri, 28 Jul 2023 18:22:53 +0000
From: Long Li <longli@...rosoft.com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: 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>, Ajay Sharma <sharmaajay@...rosoft.com>,
	"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@...hat.com" <vkuznets@...hat.com>, "ssengar@...ux.microsoft.com"
	<ssengar@...ux.microsoft.com>, "shradhagupta@...ux.microsoft.com"
	<shradhagupta@...ux.microsoft.com>
Subject: RE: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib
 driver.

> Subject: Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib
> driver.
> 
> On Fri, Jul 28, 2023 at 05:51:46PM +0000, Long Li wrote:
> > > Subject: Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support
> > > to mana ib driver.
> > >
> > > On Fri, Jul 28, 2023 at 05:07:49PM +0000, Wei Hu wrote:
> > > > Add EQ interrupt support for mana ib driver. Allocate EQs per
> > > > ucontext to receive interrupt. Attach EQ when CQ is created. Call
> > > > CQ interrupt handler when completion interrupt happens. EQs are
> > > > destroyed when ucontext is deallocated.
> > >
> > > It seems strange that interrupts would be somehow linked to a ucontext?
> > > interrupts are highly limited, you can DOS the entire system if
> > > someone abuses this.
> > >
> > > Generally I expect a properly functioning driver to use one interrupt per CPU
> core.
> >
> > Yes, MANA uses one interrupt per CPU. One interrupt is shared among
> > multiple EQs.
> 
> So you have another multiplexing layer between the interrupt and the EQ? That is
> alot of multiplexing layers..
> 
> > > You should tie the CQ to a shared EQ belong to the core that the CQ
> > > wants to have affinity to.
> >
> > The reason for using a separate EQ for a ucontext, is for preventing
> > DOS. If we use a shared EQ, a single ucontext can storm this shared EQ
> > affecting other users.
> 
> With a proper design it should not be possible. The CQ adds an entry to the EQ
> and that should be rate limited by the ability of userspace to schedule to re-arm
> the CQ.

I think DPDK user space can sometimes storm the EQ by arming the CQ from user-mode.

Please see the following code on arming the CQ from MLX4:
https://github.com/DPDK/dpdk/blob/12fcafcd62286933e6b167b14856d21f642efa5f/drivers/net/mlx4/mlx4_intr.c#L229

With a malicious DPDK user, this code can be abused to arm the CQ at extremely high rate.

Long