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

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Date:   Fri, 12 Mar 2021 05:50:34 +0900
From:   Keith Busch <kbusch@...nel.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Bjorn Helgaas <helgaas@...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 11, 2021 at 04:22:34PM -0400, Jason Gunthorpe wrote:
> On Thu, Mar 11, 2021 at 12:16:02PM -0700, Keith Busch wrote:
> > On Thu, Mar 11, 2021 at 12:17:29PM -0600, Bjorn Helgaas wrote:
> > > On Wed, Mar 10, 2021 at 03:34:01PM -0800, Alexander Duyck wrote:
> > > > 
> > > > I'm not so much worried about management software as the fact that
> > > > this is a vendor specific implementation detail that is shaping how
> > > > the kernel interfaces are meant to work. Other than the mlx5 I don't
> > > > know if there are any other vendors really onboard with this sort of
> > > > solution.
> > > 
> > > I know this is currently vendor-specific, but I thought the value
> > > proposition of dynamic configuration of VFs for different clients
> > > sounded compelling enough that other vendors would do something
> > > similar.  But I'm not an SR-IOV guy and have no vendor insight, so
> > > maybe that's not the case?
> > 
> > NVMe has a similar feature defined by the standard where a PF controller can
> > dynamically assign MSIx vectors to VFs. The whole thing is managed in user
> > space with an ioctl, though. I guess we could wire up the driver to handle it
> > through this sysfs interface too, but I think the protocol specific tooling is
> > more appropriate for nvme.
> 
> Really? Why not share a common uAPI?

We associate interrupt vectors with other dynamically assigned nvme
specific resources (IO queues), and these are not always allocated 1:1.
A common uAPI for MSIx only gets us half way to configuring the VFs for
that particular driver.
 
> Do you have a standards reference for this?

Yes, sections 5.22 and 8.5 from this spec:

  https://nvmexpress.org/wp-content/uploads/NVM-Express-1_4a-2020.03.09-Ratified.pdf

An example of open source tooling implementing this is nvme-cli's
"nvme virt-mgmt" command.