linux-kernel - Re: [PATCH net-next 0/4] net/mlx5: Memory optimizations

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Date:   Thu, 2 Dec 2021 17:28:03 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Saeed Mahameed <saeedm@...dia.com>
Cc:     Shay Drory <shayd@...dia.com>, Jiri Pirko <jiri@...dia.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 0/4] net/mlx5: Memory optimizations

On Thu, 2 Dec 2021 18:55:37 +0000 Saeed Mahameed wrote:
> On Thu, 2021-12-02 at 09:31 -0800, Jakub Kicinski wrote:
> > On Wed, 1 Dec 2021 10:22:17 +0200 Shay Drory wrote:  
> > > EQ resides in the host memory. It is RO for host driver, RW by
> > > device.
> > > When interrupt is generated EQ entry is placed by device and read
> > > by driver.
> > > It indicates about what event occurred such as CQE, async and more.  
> > 
> > I understand that. My point was the resource which is being consumed
> > here is _host_ memory. Is there precedent for configuring host memory
> > consumption via devlink resource?
> 
> it's a device resource size nonetheless, devlink resource API makes
> total sense.

I disagree. Devlink resources were originally written to partition
finite device resources. You're just sizing a queue here.

> > I'd even question whether this belongs in devlink in the first place.
> > It is not global device config in any way. If devlink represents the
> > entire device it's rather strange to have a case where main instance
> > limits a size of some resource by VFs and other endpoints can still
> > choose whatever they want.
> 
> This resource is per function instance, we have devlink instance per
> function, e.g. in the VM, there is a VF devlink instance the VM user
> can use to control own VF resources. in the PF/Hypervisor, the only
> devlink representation of the VF will be devlink port function (used
> for other purposes)
> 
> for example:
> 
> A tenant can fine-tune a resource size tailored to their needs via the
> VF's own devlink instance.

Yeah, because it's a device resource. Tenant can consume their host
DRAM in any way they find suitable.

> An admin can only control or restrict a max size of a resource for a
> given port function ( the devlink instance that represents the VF in
> the hypervisor). (note: this patchset is not about that)
> 
> > > So far no feedback by other vendors.
> > > The resources are implemented in generic way, if other vendors
> > > would
> > > like to implement them.  
> > 
> > Well, I was hoping you'd look around, but maybe that's too much to
> > ask of a vendor.  
> 
> We looked, eq is a common object among many other drivers.
> and DEVLINK_PARAM_GENERIC_ID_MAX_MACS is already a devlink generic
> param, and i am sure other vendors have limited macs per VF :) .. 
> so this applies to all vendors even if they don't advertise it.

Yeah, if you're not willing to model the Event Queue as a queue using
params seems like a better idea than abusing resources.