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

Bjorn Helgaas
2021-03-30T01:30:28Z

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