Subject: Re: [PATCH mlx5-next v1 1/5] PCI: Add sysfs callback to allow MSI-X table size change of SR-IOV VFs

From: Leon Romanovsky <[email-masked]>
Date: Wed, 13 Jan 2021 21:49:18 +0200

On Wed, Jan 13, 2021 at 10:50:52AM -0700, Alex Williamson wrote:
> On Sun, 10 Jan 2021 17:07:23 +0200
> Leon Romanovsky <[email-masked]> wrote:
>
> > From: Leon Romanovsky <[email-masked]>
> >
> > Extend PCI sysfs interface with a new callback that allows configure
> > the number of MSI-X vectors for specific SR-IO VF. This is needed
> > to optimize the performance of newly bound devices by allocating
> > the number of vectors based on the administrator knowledge of targeted VM.
> >
> > This function is applicable for SR-IOV VF because such devices allocate
> > their MSI-X table before they will run on the VMs and HW can't guess the
> > right number of vectors, so the HW allocates them statically and equally.
> >
> > The newly added /sys/bus/pci/devices/.../vf_msix_vec file will be seen
> > for the VFs and it is writable as long as a driver is not bounded to the VF.
> >
> > The values accepted are:
> >  * > 0 - this will be number reported by the VF's MSI-X capability
> >  * < 0 - not valid
> >  * = 0 - will reset to the device default value
> >
> > Signed-off-by: Leon Romanovsky <[email-masked]>
> > ---
> >  Documentation/ABI/testing/sysfs-bus-pci | 20 ++++++++
> >  drivers/pci/iov.c                       | 62 +++++++++++++++++++++++++
> >  drivers/pci/msi.c                       | 29 ++++++++++++
> >  drivers/pci/pci-sysfs.c                 |  1 +
> >  drivers/pci/pci.h                       |  2 +
> >  include/linux/pci.h                     |  8 +++-
> >  6 files changed, 121 insertions(+), 1 deletion(-)

<...>

> > +/**
> > + * pci_set_msix_vec_count - change the reported number of MSI-X vectors
> > + * This function is applicable for SR-IOV VF because such devices allocate
> > + * their MSI-X table before they will run on the VMs and HW can't guess the
> > + * right number of vectors, so the HW allocates them statically and equally.
> > + * @dev: VF device that is going to be changed
> > + * @numb: amount of MSI-X vectors
> > + **/
> > +int pci_set_msix_vec_count(struct pci_dev *dev, int numb)
> > +{
> > +	struct pci_dev *pdev = pci_physfn(dev);
> > +
> > +	if (!dev->msix_cap || !pdev->msix_cap)
> > +		return -EINVAL;
> > +
> > +	if (dev->driver || !pdev->driver ||
> > +	    !pdev->driver->sriov_set_msix_vec_count)
> > +		return -EOPNOTSUPP;
>
>
> This seems racy, don't we need to hold device_lock on both the VF and
> PF to avoid driver {un}binding races?  Does that happen implicitly
> somewhere?  Thanks,

Yes, you are right absolutely, pdev and dev are not protected here.

Thanks