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

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Date:   Mon, 29 Mar 2021 20:29:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Keith Busch <kbusch@...nel.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Saeed Mahameed <saeedm@...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 Fri, Mar 26, 2021 at 04:01:48PM -0300, Jason Gunthorpe wrote:
> On Fri, Mar 26, 2021 at 11:50:44AM -0700, Alexander Duyck wrote:
> 
> > My concern would be that we are defining the user space interface.
> > Once we have this working as a single operation I could see us having
> > to support it that way going forward as somebody will script something
> > not expecting an "offline" sysfs file, and the complaint would be that
> > we are breaking userspace if we require the use of an "offline"
> > file.
> 
> Well, we wouldn't do that. The semantic we define here is that the
> msix_count interface 'auto-offlines' if that is what is required. If
> we add some formal offline someday then 'auto-offline' would be a NOP
> when the device is offline and do the same online/offline sequence as
> today if it isn't.

Alexander, Keith, any more thoughts on this?

I think I misunderstood Greg's subdirectory comment.  We already have
directories like this:

  /sys/bus/pci/devices/0000:01:00.0/link/
  /sys/bus/pci/devices/0000:01:00.0/msi_irqs/
  /sys/bus/pci/devices/0000:01:00.0/power/

and aspm_ctrl_attr_group (for "link") is nicely done with static
attributes.  So I think we could do something like this:

  /sys/bus/pci/devices/0000:01:00.0/   # PF directory
    sriov/                             # SR-IOV related stuff
      vf_total_msix
      vf_msix_count_BB:DD.F        # includes bus/dev/fn of first VF
      ...
      vf_msix_count_BB:DD.F        # includes bus/dev/fn of last VF

And I think this could support the mlx5 model as well as the NVMe
model.

For NVMe, a write to vf_msix_count_* would have to auto-offline the VF
before asking the PF to assign the vectors, as Jason suggests above.
Before VF Enable is set, the vf_msix_count_* files wouldn't exist and
we wouldn't be able to assign vectors to VFs; IIUC that's a difference
from the NVMe interface, but maybe not a terrible one?

I'm not proposing changing nvme-cli to use this, but if the interface
is general enough to support both, that would be a good clue that it
might be able to support future devices with similar functionality.

Bjorn