netdev - Re: [RFC] PCI: Allow sysfs control over totalvfs

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Date:   Tue, 20 Sep 2016 14:36:17 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Yuval Mintz <Yuval.Mintz@...gic.com>
Cc:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        derek.chickles@...iumnetworks.com,
        Yuval Mintz <Yuval.Mintz@...iumnetworks.com>
Subject: Re: [RFC] PCI: Allow sysfs control over totalvfs

On Tue, Sep 20, 2016 at 8:49 AM, Yuval Mintz <Yuval.Mintz@...gic.com> wrote:
> [Sorry in advance if this was already discussed in the past]
>
> Some of the HW capable of SRIOV has resource limitations, where the
> PF and VFs resources are drawn from a common pool.
> In some cases, these limitations have to be considered early during
> chip initialization and can only be changed by tearing down the
> configuration and re-initializing.
> As a result, drivers for such HWs sometimes have to make unfavorable
> compromises where they reserve sufficient resources to accomadate
> the maximal number of VFs that can be created - at the expanse of
> resources that could have been used by the PF.
>
> If users were able to provide 'hints' regarding the required number
> of VFs *prior* to driver attachment, then such compromises could be
> avoided. As we already have sysfs nodes that can be queried for the
> number of totalvfs, it makes sense to let the user reduce the number
> of said totalvfs using same infrastrucure.
> Then, we can have drivers supporting SRIOV take that value into account
> when deciding how much resources to reserve, allowing the PF to benefit
> from the difference between the configuration space value and the actual
> number needed by user.
>
> Signed-off-by: Yuval Mintz <Yuval.Mintz@...iumnetworks.com>
> ---
>  drivers/pci/pci-sysfs.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index bcd10c7..c1546f8 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -449,6 +449,30 @@ static ssize_t sriov_totalvfs_show(struct device *dev,
>         return sprintf(buf, "%u\n", pci_sriov_get_totalvfs(pdev));
>  }
>
> +static ssize_t sriov_totalvfs_store(struct device *dev,
> +                                   struct device_attribute *attr,
> +                                   const char *buf, size_t count)
> +{
> +       struct pci_dev *pdev = to_pci_dev(dev);
> +       u16 max_vfs;
> +       int ret;
> +
> +       ret = kstrtou16(buf, 0, &max_vfs);
> +       if (ret < 0)
> +               return ret;
> +
> +       if (pdev->driver) {
> +               dev_info(&pdev->dev,
> +                        "Can't change totalvfs while driver is attached\n");
> +               return -EUSERS;
> +       }
> +
> +       ret = pci_sriov_set_totalvfs(pdev, max_vfs);
> +       if (ret)
> +               return ret;
> +
> +       return count;
> +}
>
>  static ssize_t sriov_numvfs_show(struct device *dev,
>                                  struct device_attribute *attr,
> @@ -516,7 +540,9 @@ static ssize_t sriov_numvfs_store(struct device *dev,
>         return count;
>  }
>
> -static struct device_attribute sriov_totalvfs_attr = __ATTR_RO(sriov_totalvfs);
> +static struct device_attribute sriov_totalvfs_attr =
> +               __ATTR(sriov_totalvfs, (S_IRUGO|S_IWUSR|S_IWGRP),
> +                      sriov_totalvfs_show, sriov_totalvfs_store);
>  static struct device_attribute sriov_numvfs_attr =
>                 __ATTR(sriov_numvfs, (S_IRUGO|S_IWUSR|S_IWGRP),
>                        sriov_numvfs_show, sriov_numvfs_store);

It would be useful to have an interface where you could increase the
number after you have decreased it.  With the interface as you have it
written that isn't an option since pci_sriov_set_totalvfs is really
only meant to strip VFs if they cannot be support by something such as
a bus limitation due to ARI not being supported.

I really think that if you need something like this you might be
better off using something like dev-link or just to figure out a way
to make your driver flexible enough to allow you to move resources
into and/or out of your PF interface if VFs are added or removed.  I
know in the case of the Intel parts we have to bounce the link when
SR-IOV is enabled because we actually go through and tear out the
queues and interrupts from the PF and then reassign all of them
between the PF and VFs before we bring the PF back up.

- Alex