netdev - Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

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Date:   Thu, 25 Mar 2021 15:28:36 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Keith Busch <kbusch@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Leon Romanovsky <leon@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-rdma@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
        Don Dutile <ddutile@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

On Thu, Mar 25, 2021 at 01:20:21PM -0500, Bjorn Helgaas wrote:
> On Thu, Mar 25, 2021 at 02:36:46PM -0300, Jason Gunthorpe wrote:
> > On Thu, Mar 25, 2021 at 12:21:44PM -0500, Bjorn Helgaas wrote:
> > 
> > > NVMe and mlx5 have basically identical functionality in this respect.
> > > Other devices and vendors will likely implement similar functionality.
> > > It would be ideal if we had an interface generic enough to support
> > > them all.
> > > 
> > > Is the mlx5 interface proposed here sufficient to support the NVMe
> > > model?  I think it's close, but not quite, because the the NVMe
> > > "offline" state isn't explicitly visible in the mlx5 model.
> > 
> > I thought Keith basically said "offline" wasn't really useful as a
> > distinct idea. It is an artifact of nvme being a standards body
> > divorced from the operating system.
> > 
> > In linux offline and no driver attached are the same thing, you'd
> > never want an API to make a nvme device with a driver attached offline
> > because it would break the driver.
> 
> I think the sticky part is that Linux driver attach is not visible to
> the hardware device, while the NVMe "offline" state *is*.  An NVMe PF
> can only assign resources to a VF when the VF is offline, and the VF
> is only usable when it is online.
> 
> For NVMe, software must ask the PF to make those online/offline
> transitions via Secondary Controller Offline and Secondary Controller
> Online commands [1].  How would this be integrated into this sysfs
> interface?

Either the NVMe PF driver tracks the driver attach state using a bus
notifier and mirrors it to the offline state, or it simply
offline/onlines as part of the sequence to program the MSI change.

I don't see why we need any additional modeling of this behavior. 

What would be the point of onlining a device without a driver?

Jason