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

Keith Busch
2021-03-30T18:11:34Z

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.