linux-kernel - Re: [v2] PCI: Add an option to control probing of VFs before enabling SR-IOV

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Message-ID: <20170411211211.GB14778@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 11 Apr 2017 16:12:11 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     bodong@...lanox.com
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, saeedm@...lanox.com,
        Eli Cohen <eli@...lanox.com>
Subject: Re: [v2] PCI: Add an option to control probing of VFs before
 enabling SR-IOV

Hi Bodong,

On Wed, Mar 22, 2017 at 05:53:58PM +0200, bodong@...lanox.com wrote:
> From: Bodong Wang <bodong@...lanox.com>
> 
> Sometimes it is not desirable to probe the virtual functions after
> SRIOV is enabled. This can save host side resource usage by VF
> instances which would be eventually probed to VMs.
> 
> Add a new PCI sysfs interface "sriov_probe_vfs" to control that
> from the PF, all current callers still retain the same functionality.
> To modify it, echo 0/n/N (disable probe) or 1/y/Y (enable probe) to
> 
> /sys/bus/pci/devices/<DOMAIN:BUS:DEVICE.FUNCTION>/sriov_probe_vfs

Is this basically the same functionality as /sys/bus/pci/drivers_autoprobe, 
but limited to a specific PF?  I.e., could we accomplish the same thing
with the following?

  # echo 0 > /sys/bus/pci/devices/DDDD:BB:dd.f/sriov_numvfs
  # echo 0 > /sys/bus/pci/drivers_autoprobe
  # echo 2 > /sys/bus/pci/devices/DDDD:BB:dd.f/sriov_numvfs
  # echo 1 > /sys/bus/pci/drivers_autoprobe

If not, can you contrast the above with drivers_autoprobe?  If we need
both, should they be named more similarly?

> Note that, the choice must be made before enabling VFs. The change
> will not take effect if VFs are already enabled. Simply, one can set
> sriov_numvfs to 0, choose whether to probe or not, and then resume
> sriov_numvfs.
> 
> Signed-off-by: Bodong Wang <bodong@...lanox.com>
> Signed-off-by: Eli Cohen <eli@...lanox.com>
> Reviewed-by: Gavin Shan <gwshan@...ux.vnet.ibm.com>