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    SubjectRe: [PATCH] PCI/IOV: update num_VFs earlier
    Hi Pierre,

    I'm really sorry, I totally missed your response.

    On Fri, Apr 26, 2019 at 10:11:54AM +0200, CREGUT Pierre IMT/OLN wrote:
    > I also initially thought that kobject_uevent generated the netlink event
    > but this is not the case. This is generated by the specific driver in use.
    > For the Intel i40e driver, this is the call to i40e_do_reset_safe in
    > i40e_pci_sriov_configure that sends the event.
    > It is followed by i40e_pci_sriov_enable that calls i40e_alloc_vfs that
    > finally calls the generic pci_enable_sriov function.
    >
    > So the proposed patch works well for the i40e driver (x710 cards) because
    > the update to num_VFs is fast enough to be committed before the event is
    > received. It may not work with other cards. The same is true for the zero
    > value and there is no guarantee for other cards.
    >
    > The clean solution would be to lock the device in sriov_numvfs_show.
    > I guess that there are good reasons why locks have been avoided
    > in sysfs getter functions so let us explore other approaches.
    >
    > We can either return a "not settled" value (-1) or (probably better)
    > do not return a value but an error (-EAGAIN returned by the show
    > function).
    >
    > To distinguish this "not settled" situation we can either:
    > * overload the meaning of num_VFs (eg make it negative)
    > but it is an unsigned short.
    > * add a bool to pci_sriov struct (rather simple but modifies a well
    > established structure).
    > * use the fact that not_settled => device is locked and use
    > mutex_is_locked as an over approximation.
    >
    > The later is not perfect but requires minimal changes to
    > sriov_numvfs_show:
    >
    >  if (mutex_is_locked(&dev->mutex))
    > return -EAGAIN;

    I think this looks like a good resolution.

    > In all cases, the device could be locked or the boolean set just
    > after the test. But I don't think there is a case where causality
    > would be violated.Thank you in advance for your recommendations. I will
    > update the patch according to your instructions.
    >
    > Le 06/04/2019 à 00:33, Bjorn Helgaas a écrit :
    > > On Fri, Mar 29, 2019 at 09:00:58AM +0100, Pierre Crégut wrote:
    > > > Ensure that iov->num_VFs is set before a netlink message is sent
    > > > when the number of VFs is changed. Only the path for num_VFs > 0
    > > > is affected. The path for num_VFs = 0 is already correct.
    > > >
    > > > Monitoring programs can relie on netlink messages to track interface
    > > > change and query their state in /sys. But when sriov_numvfs is set to a
    > > > positive value, the netlink message is sent before the value is available
    > > > in sysfs. The value read after the message is received is always zero.
    > > Thanks, Pierre! Can you clue me in on where exactly the connection
    > > from sriov_enable() to netlink is?
    > >
    > > I see one side of the race is with sriov_numvfs_show(), but I don't
    > > know where the netlink message is sent. Is that connected with the
    > > kobject_uevent(KOBJ_CHANGE)?
    > >
    > > One thing this would help with is figuring out exactly how *much*
    > > earlier we need to set iov->num_VFs. It looks like the current patch
    > > sets it before we actually enable the VFs, so a user could read
    > > /sys/.../sriov_numvfs and get the wrong value. Of course, that's
    > > unavoidable; the question is whether it's OK to get the new value
    > > *before* it actually takes effect, or whether we want to return a
    > > stale value until after it takes effect.
    > >
    > > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=202991
    > > > Signed-off-by: Pierre Crégut <pierre.cregut@orange.com>
    > > > ---
    > > > note: the behaviour can be tested with the following shell script also
    > > > available on the bugzilla (d being the phy device name):
    > > >
    > > > ip monitor dev $d | grep --line-buffered "^[0-9]*:" | \
    > > > while read line; do cat /sys/class/net/$d/device/sriov_numvfs; done
    > > >
    > > > drivers/pci/iov.c | 3 ++-
    > > > 1 file changed, 2 insertions(+), 1 deletion(-)
    > > >
    > > > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
    > > > index 3aa115ed3a65..a9655c10e87f 100644
    > > > --- a/drivers/pci/iov.c
    > > > +++ b/drivers/pci/iov.c
    > > > @@ -351,6 +351,7 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
    > > > goto err_pcibios;
    > > > }
    > > > + iov->num_VFs = nr_virtfn;
    > > > pci_iov_set_numvfs(dev, nr_virtfn);
    > > > iov->ctrl |= PCI_SRIOV_CTRL_VFE | PCI_SRIOV_CTRL_MSE;
    > > > pci_cfg_access_lock(dev);
    > > > @@ -363,7 +364,6 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
    > > > goto err_pcibios;
    > > > kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
    > > > - iov->num_VFs = nr_virtfn;
    > > > return 0;
    > > > @@ -379,6 +379,7 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
    > > > if (iov->link != dev->devfn)
    > > > sysfs_remove_link(&dev->dev.kobj, "dep_link");
    > > > + iov->num_VFs = 0;
    > > > pci_iov_set_numvfs(dev, 0);
    > > > return rc;
    > > > }
    > > > --
    > > > 2.17.1
    > > >

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