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 08:09:38 +0200

On Tue, Jan 12, 2021 at 01:59:51PM -0800, Alexander Duyck wrote:
> On Mon, Jan 11, 2021 at 10:39 PM Leon Romanovsky <[email-masked]> wrote:
> >
> > On Mon, Jan 11, 2021 at 11:30:33AM -0800, Alexander Duyck wrote:
> > > On Sun, Jan 10, 2021 at 7:12 AM 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(-)
> > > >
> > > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > > index 25c9c39770c6..05e26e5da54e 100644
> > > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > > @@ -375,3 +375,23 @@ Description:
> > > >                 The value comes from the PCI kernel device state and can be one
> > > >                 of: "unknown", "error", "D0", D1", "D2", "D3hot", "D3cold".
> > > >                 The file is read only.
> > > > +
> > > > +What:          /sys/bus/pci/devices/.../vf_msix_vec
> > >
> > > So the name for this doesn't seem to match existing SR-IOV naming.  It
> > > seems like this should probably be something like sriov_vf_msix_count
> > > in order to be closer to the actual naming of what is being dealt
> > > with.
> >
> > I'm open for suggestions. I didn't use sriov_vf_msix_count because it
> > seems too long for me.
> >
> > >
> > > > +Date:          December 2020
> > > > +Contact:       Leon Romanovsky <[email-masked]>
> > > > +Description:
> > > > +               This file is associated with the SR-IOV VFs.
> > > > +               It allows configuration of the number of MSI-X vectors for
> > > > +               the VF. This is needed to optimize performance of newly bound
> > > > +               devices by allocating the number of vectors based on the
> > > > +               administrator knowledge of targeted VM.
> > > > +
> > > > +               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
> > > > +
> > > > +               The file is writable if the PF is bound to a driver that
> > > > +               supports the ->sriov_set_msix_vec_count() callback and there
> > > > +               is no driver bound to the VF.
> > > > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > > > index 4afd4ee4f7f0..42c0df4158d1 100644
> > > > --- a/drivers/pci/iov.c
> > > > +++ b/drivers/pci/iov.c
> > > > @@ -31,6 +31,7 @@ int pci_iov_virtfn_devfn(struct pci_dev *dev, int vf_id)
> > > >         return (dev->devfn + dev->sriov->offset +
> > > >                 dev->sriov->stride * vf_id) & 0xff;
> > > >  }
> > > > +EXPORT_SYMBOL(pci_iov_virtfn_devfn);
> > > >
> > > >  /*
> > > >   * Per SR-IOV spec sec 3.3.10 and 3.3.11, First VF Offset and VF Stride may
> > > > @@ -426,6 +427,67 @@ const struct attribute_group sriov_dev_attr_group = {
> > > >         .is_visible = sriov_attrs_are_visible,
> > > >  };
> > > >
> > > > +#ifdef CONFIG_PCI_MSI
> > > > +static ssize_t vf_msix_vec_show(struct device *dev,
> > > > +                               struct device_attribute *attr, char *buf)
> > > > +{
> > > > +       struct pci_dev *pdev = to_pci_dev(dev);
> > > > +       int numb = pci_msix_vec_count(pdev);
> > > > +       struct pci_dev *pfdev;
> > > > +
> > > > +       if (numb < 0)
> > > > +               return numb;
> > > > +
> > > > +       pfdev = pci_physfn(pdev);
> > > > +       if (!pfdev->driver || !pfdev->driver->sriov_set_msix_vec_count)
> > > > +               return -EOPNOTSUPP;
> > > > +
> > >
> > > This doesn't make sense to me. You are getting the vector count for
> > > the PCI device and reporting that. Are you expecting to call this on
> > > the PF or the VFs? It seems like this should be a PF attribute and not
> > > be called on the individual VFs.
> >
> > We had this discussion over v0 variant of this series.
> > https://lore.kernel.org/linux-pci/20210108072525.GB31158@unreal/
> >
> > This is per-VF property, but this VF is not bounded to the driver so you
> > need some way to convey new number to the HW, so it will update PCI value.
> >
> > You must change/update this field after VF is created, because all SR-IOV VFs
> > are created at the same time. The operator (administrator/orchestration
> > software/e.t.c) will know the right amount of MSI-X vectors right before
> > he will bind this VF to requested VM.
> >
> > It means that extending PF sysfs to get both VF index and count will
> > look very unfriendly for the users.
> >
> > The PF here is an anchor to the relevant driver.
>
> Yes, but the problem is you are attempting to do it after a driver may
> have already bound itself to the VF device. This setup works for the
> direct assigned VF case, however if the VF drivers are running on the
> host then this gets ugly as the driver may already be up and running.

Please take a look on the pci_set_msix_vec_count() implementation, it
checks that VF is not probed yet. I outlined this requirement almost
in all my responses and commit messages.

So no, it is not possible to set MSI-X vectors to already bound device.

>
> Also I am not a big fan of the VF groping around looking for a PF
> interface as it means the interface will likely be exposed in the
> guest as well, but it just won't work.

If you are referring to VF exposed to the VM, so in this case VF must be
bound too vfio driver, or any other driver, and won't allow MSI-X change.
If you are referring to PF exposed to the VM, it is very unlikely scenario
in real world and reserved for braves among us. Even in this case, the
set MSI-X won't work, because PF will be connected to the hypervisor driver
that doesn't support set_msix.

So both cases are handled.

>
> > >
> > > If you are calling this on the VFs then it doesn't really make any
> > > sense anyway since the VF is not a "VF PCI dev representor" and
> > > shouldn't be treated as such. In my opinion if we are going to be
> > > doing per-port resource limiting that is something that might make
> > > more sense as a part of the devlink configuration for the VF since the
> > > actual change won't be visible to an assigned device.
> >
> > https://lore.kernel.org/linux-pci/20210112061535.GB4678@unreal/
>
> So the question I would have is if we are spawning the VFs and
> expecting them to have different configs or the same configuration?

By default, they have same configuration.

> I'm assuming in your case you are looking for a different
> configuration per port. Do I have that correct?

No, per-VF as represents one device in the PCI world. For example, mlx5
can have more than one physical port.

>
> Where this gets ugly is that SR-IOV assumes a certain uniformity per
> VF so doing a per-VF custom limitation gets ugly pretty quick.

I don't find any support for this "uniformity" claim in the PCI spec.

> I wonder if it would make more sense if we are going this route to just
> define a device-tree like schema that could be fed in to enable VFs
> instead of just using echo X > sriov_numvfs and then trying to fix
> things afterwards. Then you could define this and other features that
> I am sure you would need in the future via json-schema like is done in
> device-tree and write it once enabling the set of VFs that you need.

Sorry, but this is overkill, it won't give us much and it doesn't fit
the VF usage model at all.

Right now, all heavy users of SR-IOV are creating many VFs up to the maximum.
They do it with autoprobe disabled, because it is too time consuming to wait
till all VFs probe themselves and unbind them later.

After that, they wait for incoming request to provision VM on VF, they set MAC
address, change MSI-X according to VM properties and bind that VF to new VM.

So MSI-X change is done after VFs were created.

>
> Given that SR-IOV isn't meant to be tweaked dynamically it seems like
> that would be a much better fit for most users as then you can just
> modify the schema and reload it which would probably be less effort
> than having to manually redo all the commands needed to setup the VFs
> using this approach if you are having to manually update each VF.

Quite opposite it true. First users use orchestration software to manage it.
Second, you will need to take care of already bound device.

>
> > >
> > > > +       return sprintf(buf, "%d\n", numb);
> > > > +}
> > > > +
> > > > +static ssize_t vf_msix_vec_store(struct device *dev,
> > > > +                                struct device_attribute *attr, const char *buf,
> > > > +                                size_t count)
> > > > +{
> > > > +       struct pci_dev *vf_dev = to_pci_dev(dev);
> > > > +       int val, ret;
> > > > +
> > > > +       ret = kstrtoint(buf, 0, &val);
> > > > +       if (ret)
> > > > +               return ret;
> > > > +
> > > > +       ret = pci_set_msix_vec_count(vf_dev, val);
> > > > +       if (ret)
> > > > +               return ret;
> > > > +
> > > > +       return count;
> > > > +}
> > > > +static DEVICE_ATTR_RW(vf_msix_vec);
> > > > +#endif
> > > > +
> > > > +static struct attribute *sriov_vf_dev_attrs[] = {
> > > > +#ifdef CONFIG_PCI_MSI
> > > > +       &dev_attr_vf_msix_vec.attr,
> > > > +#endif
> > > > +       NULL,
> > > > +};
> > > > +
> > > > +static umode_t sriov_vf_attrs_are_visible(struct kobject *kobj,
> > > > +                                         struct attribute *a, int n)
> > > > +{
> > > > +       struct device *dev = kobj_to_dev(kobj);
> > > > +
> > > > +       if (dev_is_pf(dev))
> > > > +               return 0;
> > > > +
> > > > +       return a->mode;
> > > > +}
> > > > +
> > > > +const struct attribute_group sriov_vf_dev_attr_group = {
> > > > +       .attrs = sriov_vf_dev_attrs,
> > > > +       .is_visible = sriov_vf_attrs_are_visible,
> > > > +};
> > > > +
> > > >  int __weak pcibios_sriov_enable(struct pci_dev *pdev, u16 num_vfs)
> > > >  {
> > > >         return 0;
> > > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > > index 3162f88fe940..20705ca94666 100644
> > > > --- a/drivers/pci/msi.c
> > > > +++ b/drivers/pci/msi.c
> > > > @@ -991,6 +991,35 @@ int pci_msix_vec_count(struct pci_dev *dev)
> > > >  }
> > > >  EXPORT_SYMBOL(pci_msix_vec_count);
> > > >
> > > > +/**
> > > > + * 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;
> > > > +
> > > > +       if (numb < 0)
> > > > +               /*
> > > > +                * We don't support negative numbers for now,
> > > > +                * but maybe in the future it will make sense.
> > > > +                */
> > > > +               return -EINVAL;
> > > > +
> > > > +       return pdev->driver->sriov_set_msix_vec_count(dev, numb);
> > > > +}
> > > > +
> > >
> > > If you are going to have a set operation for this it would make sense
> > > to have a get operation. Your show operation seems unbalanced since
> > > you are expecting to call it on the VF directly which just seems
> > > wrong.
> >
> > There is already get operator - pci_msix_vec_count().
> > The same as above, PF is an anchor for the driver. VF doesn't have
> > driver yet and we can't write directly to this PCI field - it is read-only.
>
> That returns the maximum. I would want to know what the value is that
> you wrote here.

It returns the value from the device and not maximum.

>
> Also being able to change this once the device already exists is kind
> of an ugly setup as I would imagine there will be cases where the
> device has already partitioned out the vectors that it wanted.

It is not possible, because all VFs starts with some value. Normal FW
won't allow you to decrease vector count below certain limit.

So VFs are always operable.

>
> > >
> > > >  static int __pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries,
> > > >                              int nvec, struct irq_affinity *affd, int flags)
> > > >  {
> > > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > > index fb072f4b3176..0af2222643c2 100644
> > > > --- a/drivers/pci/pci-sysfs.c
> > > > +++ b/drivers/pci/pci-sysfs.c
> > > > @@ -1557,6 +1557,7 @@ static const struct attribute_group *pci_dev_attr_groups[] = {
> > > >         &pci_dev_hp_attr_group,
> > > >  #ifdef CONFIG_PCI_IOV
> > > >         &sriov_dev_attr_group,
> > > > +       &sriov_vf_dev_attr_group,
> > > >  #endif
> > > >         &pci_bridge_attr_group,
> > > >         &pcie_dev_attr_group,
> > > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > > index 5c59365092fa..1fd273077637 100644
> > > > --- a/drivers/pci/pci.h
> > > > +++ b/drivers/pci/pci.h
> > > > @@ -183,6 +183,7 @@ extern unsigned int pci_pm_d3hot_delay;
> > > >
> > > >  #ifdef CONFIG_PCI_MSI
> > > >  void pci_no_msi(void);
> > > > +int pci_set_msix_vec_count(struct pci_dev *dev, int numb);
> > > >  #else
> > > >  static inline void pci_no_msi(void) { }
> > > >  #endif
> > > > @@ -502,6 +503,7 @@ resource_size_t pci_sriov_resource_alignment(struct pci_dev *dev, int resno);
> > > >  void pci_restore_iov_state(struct pci_dev *dev);
> > > >  int pci_iov_bus_range(struct pci_bus *bus);
> > > >  extern const struct attribute_group sriov_dev_attr_group;
> > > > +extern const struct attribute_group sriov_vf_dev_attr_group;
> > > >  #else
> > > >  static inline int pci_iov_init(struct pci_dev *dev)
> > > >  {
> > > > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > > > index b32126d26997..a17cfc28eb66 100644
> > > > --- a/include/linux/pci.h
> > > > +++ b/include/linux/pci.h
> > > > @@ -856,6 +856,8 @@ struct module;
> > > >   *             e.g. drivers/net/e100.c.
> > > >   * @sriov_configure: Optional driver callback to allow configuration of
> > > >   *             number of VFs to enable via sysfs "sriov_numvfs" file.
> > > > + * @sriov_set_msix_vec_count: Driver callback to change number of MSI-X vectors
> > > > + *              exposed by the sysfs "vf_msix_vec" entry.
> > >
> > > Hopefully it is doing more than just changing the displayed sysfs
> > > value. What is the effect of changing that value on the actual system
> > > state? I'm assuming this is some limit that is enforced by the PF or
> > > the device firmware?
> >
> > The whole purpose of this interface it to change read-only field and to
> > prevent any possible complications, this field is possible to change
> > only for unprobed device.
>
> Calling this out somewhere and documenting that might be useful.
> Otherwise it is very easy to overlook the absence of a "!" when
> reviewing a set of patches such as these.

It is documented in the Documentation file, but I will add extra comment.

>
> > The limit checks are done in FW as it is the entity that manages SR-IOV PCI
> > config space. This is true for mlx5, other drivers can do their checks in the SW.
>
> I get that. The problem is this is pushing configuration into firmware
> that may not be visible to the kernel  or software. Thus why I was
> asking how you would display the value once it is written.

pci_msix_vec_count() and "lspci -vv ... | grep MSI-X" should give new value.

Thanks