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

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Date:   Wed, 31 Mar 2021 03:10:39 +0900
From:   Keith Busch <kbusch@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        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 Mon, Mar 29, 2021 at 08:29:49PM -0500, Bjorn Helgaas wrote:
> 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?

Yes, that's fine, nvme can handle this flow. It is a little easier to
avoid nvme user error if we could mainpulate the counts prior to VF Enable,
but it's really not a problem this way either.

I think it's reasonable for nvme to subscribe to this interface, but I
will have to defer to someone with capable nvme devices to implement it.