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

Bjorn Helgaas
2021-03-25T18:21:11Z

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?

> So I think it is good as is (well one of the 8 versions anyhow).
>
> Keith didn't go into detail why the queue allocations in nvme were any
> different than the queue allocations in mlx5. I expect they can
> probably work the same where the # of interrupts is an upper bound on
> the # of CPUs that can get queues and the device, once instantiated,
> could be configured for the number of queues to actually operate, if
> it wants.

I don't really care about the queue allocations. I don't think we
need to solve those here; we just need to make sure that what we do
here doesn't preclude NVMe queue allocations.

Bjorn

[1] NVMe 1.4a, sec 5.22