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

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Message-ID: <20210927144756.GA643630@bhelgaas>
Date:   Mon, 27 Sep 2021 09:47:56 -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 Mon, Sep 27, 2021 at 02:55:24PM +0300, Leon Romanovsky wrote:
> On Sun, Sep 26, 2021 at 03:23:41PM -0500, Bjorn Helgaas wrote:
> > On Sun, Sep 26, 2021 at 09:36:49AM +0300, Leon Romanovsky wrote:
> > > 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@...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).
> > > > > ...
> > > > 
> > > > > > 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.
> > 
> > IMHO you should resolve it by using the new interface.  Better
> > performing, unambiguous regardless of how many VFs the device
> > supports.  What's the down side?
> 
> I don't see any. My previous answer worth to be written.
> "No problem, we will make it more clear with this new function".

Great, sorry I missed that nuance :)