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

[Leon Romanovsky <leon@kernel.org>]

On Sat, Sep 25, 2021 at 12:41:15PM -0500, Bjorn Helgaas wrote:
> On Sat, Sep 25, 2021 at 01:10:39PM +0300, Leon Romanovsky wrote:
> > On Fri, Sep 24, 2021 at 08:08:45AM -0500, Bjorn Helgaas wrote:
> > > 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@nvidia.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@nvidia.com>
> > > > > > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > > > > > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > > > > 
> > > > > Acked-by: Bjorn Helgaas <bhelgaas@google.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).
> > ...
> 
> > > 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.
> > 
> > In mlx5_core_sriov_set_msix_vec_count(), we receive VF that is connected
> > to PF which has "struct mlx5_core_dev". My expectation is that they share
> > same bus as that PF was the one who created VFs. The mlx5 devices supports
> > upto 256 VFs and it is far below the bus split mentioned in PCI spec.
> > 
> > How can VF and their respective PF have different bus numbers?
> 
> See PCIe r5.0, sec 9.2.1.2.  For example,
> 
>   PF 0 on bus 20
>     First VF Offset   1
>     VF Stride         1
>     NumVFs          511
>   VF 0,1   through VF 0,255 on bus 20
>   VF 0,256 through VF 0,511 on bus 21
> 
> This is implemented in pci_iov_add_virtfn(), which computes the bus
> number and devfn from the VF ID.
> 
> pci_iov_virtfn_devfn(VF 0,1) == pci_iov_virtfn_devfn(VF 0,256), so if
> the user writes to sriov_vf_msix_count for VF 0,256, it looks like
> we'll call mlx5_set_msix_vec_count() for VF 0,1 instead of VF 0,256.

This is PCI spec split that I mentioned.

> 
> The spec encourages devices that require no more than 256 devices to
> locate them all on the same bus number (PCIe r5.0, sec 9.1), so if you
> only have 255 VFs, you may avoid the problem.
> 
> But in mlx5_core_sriov_set_msix_vec_count(), it's not obvious that it
> is safe to assume the bus number is the same.

No problem, we will make it more clear.

> 
> Bjorn