Subject: Re: [PATCH mlx5-next 1/4] PCI: Configure number of MSI-X vectors for SR-IOV VFs

From: Leon Romanovsky <[email-masked]>
Date: Sun, 10 Jan 2021 10:22:06 +0200

On Thu, Jan 07, 2021 at 06:57:21PM -0600, Bjorn Helgaas wrote:
> [+cc Alex, Don]
>
> This patch does not actually *configure* the number of vectors, so the
> subject is not quite accurate.  IIUC, this patch adds a sysfs file
> that can be used to configure the number of vectors.  The subject
> should mention the sysfs connection.

I'll do:
"PCI: Add sysfs callback to allow MSI-X table size change of SR-IOV VFs"

>
> On Sun, Jan 03, 2021 at 10:24:37AM +0200, Leon Romanovsky wrote:
> > From: Leon Romanovsky <[email-masked]>
> >
> > This function is applicable for SR-IOV VFs because such devices allocate
> > their MSI-X table before they will run on the targeted hardware and they
> > can't guess the right amount of vectors.
>
> This sentence doesn't quite have enough context to make sense to me.
> Per PCIe r5.0, sec 9.5.1.2, I think PFs and VFs have independent MSI-X
> Capabilities.  What is the connection between the PF MSI-X and the VF
> MSI-X?

Right, PF and VF have different capabilities, but MSI-X vectors are
limited resource by the HW and the device has pool of such vectors
to distribute to the VFs.

The connection between PF and VF is a logical one. The PF exists and bounded
to the driver, so have an ability to actually write to the HW and change VF
configuration before driver bounded to it.

>
> The MSI-X table sizes should be determined by the Table Size in the
> Message Control register.  Apparently we write a VF's Table Size
> before a driver is bound to the VF?  Where does that happen?

The table size is set by the HW when SR-IOV is enabled and VFs are created.
echo num_sriov > /sys/bus/pci/devices/.../sriov_numvfs
.... at this point VFs have this table set, but not used yet.

The driver will read this table when it enables MSI-X:
 pci_enable_msix_range
  __pci_enable_msix_range
   __pci_enable_msix
    pci_msix_vec_count

>
> "Before they run on the targeted hardware" -- do you mean before the
> VF is passed through to a guest virtual machine?  You mention "target
> VM" below, which makes more sense to me.  VFs don't "run"; they're not
> software.  I apologize for not being an expert in the use of VFs.

Yes, "target" == "VM".

>
> Please mention the sysfs path in the commit log.

I'll do.

>
> > Signed-off-by: Leon Romanovsky <[email-masked]>
> > ---
> >  Documentation/ABI/testing/sysfs-bus-pci | 16 +++++++
> >  drivers/pci/iov.c                       | 57 +++++++++++++++++++++++++
> >  drivers/pci/msi.c                       | 30 +++++++++++++
> >  drivers/pci/pci-sysfs.c                 |  1 +
> >  drivers/pci/pci.h                       |  1 +
> >  include/linux/pci.h                     |  8 ++++
> >  6 files changed, 113 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > index 25c9c39770c6..30720a9e1386 100644
> > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > @@ -375,3 +375,19 @@ 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
> > +Date:		December 2020
> > +Contact:	Leon Romanovsky <[email-masked]>
> > +Description:
> > +		This file is associated with the SR-IOV VFs. It allows overwrite
> > +		the amount of MSI-X vectors for that VF. This is needed to optimize
> > +		performance of newly bounded devices by allocating the number of
> > +		vectors based on the internal knowledge of targeted VM.
>
> s/allows overwrite/allows configuration of/
> s/for that/for the/
> s/amount of/number of/
> s/bounded/bound/

I changed it, thanks

>
> What "internal knowledge" is this?  AFAICT this would have to be some
> user-space administration knowledge, not anything internal to the
> kernel.

Yes, it is not internal to the kernel, but administrator knowledge.
In our case, it is orchestration software that allocates such VFs to the
users.

>
> > +		The values accepted are:
> > +		 * > 0 - this will be number reported by the PCI VF's PCIe MSI-X capability.
>
> s/PCI// (it's obvious we're talking about PCI here)
> s/PCIe// (MSI-X is not PCIe-specific, and there's no need to mention
> it at all)

Done

>
> > +		 * < 0 - not valid
> > +		 * = 0 - will reset to the device default value
> > +
> > +		The file is writable if no driver is bounded.
>
> From the code, it looks more like this:
>
>   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.

I added it to the description.

>
> Please wrap all of this to fit in 80 columns like the rest of the file.

Fixed

>
> > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > index 4afd4ee4f7f0..0f8c570361fc 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 +427,62 @@ 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);
> > +
> > +	if (numb < 0)
> > +		return numb;
> > +
> > +	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..0bcd705487d9 100644
> > --- a/drivers/pci/msi.c
> > +++ b/drivers/pci/msi.c
> > @@ -991,6 +991,36 @@ 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.
>
> Drop period at end, as other kernel doc in this file does.

Done

>
> > + * This function is applicable for SR-IOV VFs because such devices allocate
> > + * their MSI-X table before they will run on the targeted hardware and they
> > + * can't guess the right amount of vectors.
> > + * @dev: VF device that is going to be changed.
> > + * @numb: amount of MSI-X vectors.
>
> Rewrite the "such devices allocate..." part based on the questions in
> the commit log.  Same with "targeted hardware."
>
> s/amount of/number of/
> Drop periods at end of parameter descriptions.

Done

>
> > + **/
> > +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);
>
> So we write to a VF sysfs file, get here and look up the PF, call a PF
> driver callback with the VF as an argument, the callback (at least for
> mlx5) looks up the PF from the VF, then does some mlx5-specific magic
> to the PF that influences the VF somehow?

Right, because HW already created VFs, the PF driver is aware of them,
so it simply says to the FW that specific VF should have different
value in their table size.

>
> Help me connect the dots here.  Is this required because of something
> peculiar to mlx5, or is something like this required for all SR-IOV
> devices because of the way the PCIe spec is written?

The second one is correct, there is nothing mlx5 specific in it.
This is a combination of the spec together with Linux SR-IOV implementation
logic.

First, PCI spec has one single bit to enable/disable all VFs at the same time
without ability to dynamically add/delete. It means all SR-IOV HW in the world
will do the same: split internal MSI-X pool equally or by any other same logic.
This is needed so lspci right after VF created will give proper values in the
MSI-X section.

Second, Linux followed the spec and implemented same allocation model and separated
it by the layers, at the time driver probes, it should have all PCI config ready in
the PCI level. It means even change of MSI-X inside VF during driver VF init and
driver reload later can be potentially problematic.

>
> > +}
> > +EXPORT_SYMBOL(pci_set_msix_vec_count);
> > +
> >  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..46396a5da2d9 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -502,6 +502,7 @@ 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..1acba40a1b1b 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.
> >   * @err_handler: See Documentation/PCI/pci-error-recovery.rst
> >   * @groups:	Sysfs attribute groups.
> >   * @driver:	Driver model structure.
> > @@ -871,6 +873,7 @@ struct pci_driver {
> >  	int  (*resume)(struct pci_dev *dev);	/* Device woken up */
> >  	void (*shutdown)(struct pci_dev *dev);
> >  	int  (*sriov_configure)(struct pci_dev *dev, int num_vfs); /* On PF */
> > +	int  (*sriov_set_msix_vec_count)(struct pci_dev *vf, int msix_vec_count); /* On PF */
> >  	const struct pci_error_handlers *err_handler;
> >  	const struct attribute_group **groups;
> >  	struct device_driver	driver;
> > @@ -1464,6 +1467,7 @@ struct msix_entry {
> >  int pci_msi_vec_count(struct pci_dev *dev);
> >  void pci_disable_msi(struct pci_dev *dev);
> >  int pci_msix_vec_count(struct pci_dev *dev);
> > +int pci_set_msix_vec_count(struct pci_dev *dev, int numb);
>
> This patch adds the pci_set_msix_vec_count() definition in pci/msi.c
> and a call in pci/iov.c.  It doesn't need to be declared in
> include/linux/pci.h or exported.  It can be declared in
> drivers/pci/pci.h.

I changed it, thanks

>
> >  void pci_disable_msix(struct pci_dev *dev);
> >  void pci_restore_msi_state(struct pci_dev *dev);
> >  int pci_msi_enabled(void);
> > @@ -2402,6 +2406,10 @@ static inline bool pci_is_thunderbolt_attached(struct pci_dev *pdev)
> >  void pci_uevent_ers(struct pci_dev *pdev, enum  pci_ers_result err_type);
> >  #endif
> >
> > +#ifdef CONFIG_PCI_IOV
> > +int pci_iov_virtfn_devfn(struct pci_dev *dev, int vf_id);
> > +#endif
>
> pci_iov_virtfn_devfn() is already declared in this file.

Sorry for that.

>
> >  /* Provide the legacy pci_dma_* API */
> >  #include <linux/pci-dma-compat.h>
> >
> > --
> > 2.29.2
> >