netdev - Re: [PATCH mlx5-next 1/7] PCI/IOV: Provide internal VF index

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Date:   Fri, 24 Sep 2021 08:08:45 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        Saeed Mahameed <saeedm@...dia.com>,
        Yishai Hadas <yishaih@...dia.com>
Subject: Re: [PATCH mlx5-next 1/7] PCI/IOV: Provide internal VF index

On Thu, Sep 23, 2021 at 09:35:32AM +0300, Leon Romanovsky wrote:
> On Wed, Sep 22, 2021 at 04:59:30PM -0500, Bjorn Helgaas wrote:
> > On Wed, Sep 22, 2021 at 01:38:50PM +0300, Leon Romanovsky wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > 
> > > The PCI core uses the VF index internally, often called the vf_id,
> > > during the setup of the VF, eg pci_iov_add_virtfn().
> > > 
> > > This index is needed for device drivers that implement live migration
> > > for their internal operations that configure/control their VFs.
> > >
> > > Specifically, mlx5_vfio_pci driver that is introduced in coming patches
> > > from this series needs it and not the bus/device/function which is
> > > exposed today.
> > > 
> > > Add pci_iov_vf_id() which computes the vf_id by reversing the math that
> > > was used to create the bus/device/function.
> > > 
> > > Signed-off-by: Yishai Hadas <yishaih@...dia.com>
> > > Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> > > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > 
> > Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > 
> > mlx5_core_sriov_set_msix_vec_count() looks like it does basically the
> > same thing as pci_iov_vf_id() by iterating through VFs until it finds
> > one with a matching devfn (although it *doesn't* check for a matching
> > bus number, which seems like a bug).
> > 
> > Maybe that should use pci_iov_vf_id()?
> 
> Yes, I gave same comment internally and we decided to simply reduce the
> amount of changes in mlx5_core to have less distractions and submit as a
> followup. Most likely will add this hunk in v1.

I guess it backfired as far as reducing distractions, because now it
just looks like a job half-done.

And it still looks like the existing code is buggy.  This is called
via sysfs, so if the PF is on bus X and the user writes to
sriov_vf_msix_count for a VF on bus X+1, it looks like
mlx5_core_sriov_set_msix_vec_count() will set the count for the wrong
VF.

Bjorn