netdev - [PATCH V2 mlx5-next 04/14] PCI/IOV: Add pci_iov_get_pf_drvdata() to allow VF reaching the drvdata of a PF

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Message-ID: <20211019105838.227569-5-yishaih@nvidia.com>
Date:   Tue, 19 Oct 2021 13:58:28 +0300
From:   Yishai Hadas <yishaih@...dia.com>
To:     <alex.williamson@...hat.com>, <bhelgaas@...gle.com>,
        <jgg@...dia.com>, <saeedm@...dia.com>
CC:     <linux-pci@...r.kernel.org>, <kvm@...r.kernel.org>,
        <netdev@...r.kernel.org>, <kuba@...nel.org>, <leonro@...dia.com>,
        <kwankhede@...dia.com>, <mgurtovoy@...dia.com>,
        <yishaih@...dia.com>, <maorg@...dia.com>
Subject: [PATCH V2 mlx5-next 04/14] PCI/IOV: Add pci_iov_get_pf_drvdata() to allow VF reaching the drvdata of a PF

From: Jason Gunthorpe <jgg@...dia.com>

There are some cases where a SR-IOV VF driver will need to reach into and
interact with the PF driver. This requires accessing the drvdata of the PF.

Provide a function pci_iov_get_pf_drvdata() to return this PF drvdata in a
safe way. Normally accessing a drvdata of a foreign struct device would be
done using the device_lock() to protect against device driver
probe()/remove() races.

However, due to the design of pci_enable_sriov() this will result in a
ABBA deadlock on the device_lock as the PF's device_lock is held during PF
sriov_configure() while calling pci_enable_sriov() which in turn holds the
VF's device_lock while calling VF probe(), and similarly for remove.

This means the VF driver can never obtain the PF's device_lock.

Instead use the implicit locking created by pci_enable/disable_sriov(). A
VF driver can access its PF drvdata only while its own driver is attached,
and the PF driver can control access to its own drvdata based on when it
calls pci_enable/disable_sriov().

To use this API the PF driver will setup the PF drvdata in the probe()
function. pci_enable_sriov() is only called from sriov_configure() which
cannot happen until probe() completes, ensuring no VF races with drvdata
setup.

For removal, the PF driver must call pci_disable_sriov() in its remove
function before destroying any of the drvdata. This ensures that all VF
drivers are unbound before returning, fencing concurrent access to the
drvdata.

The introduction of a new function to do this access makes clear the
special locking scheme and the documents the requirements on the PF/VF
drivers using this.

Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
Signed-off-by: Leon Romanovsky <leonro@...dia.com>
Signed-off-by: Yishai Hadas <yishaih@...dia.com>
---
 drivers/pci/iov.c   | 29 +++++++++++++++++++++++++++++
 include/linux/pci.h |  7 +++++++
 2 files changed, 36 insertions(+)

diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
index e7751fa3fe0b..8c724bc134c7 100644
--- a/drivers/pci/iov.c
+++ b/drivers/pci/iov.c
@@ -47,6 +47,35 @@ int pci_iov_vf_id(struct pci_dev *dev)
 }
 EXPORT_SYMBOL_GPL(pci_iov_vf_id);

+/**
+ * pci_iov_get_pf_drvdata - Return the drvdata of a PF
+ * @dev - VF pci_dev
+ * @pf_driver - Device driver required to own the PF
+ *
+ * This must be called from a context that ensures that a VF driver is attached.
+ * The value returned is invalid once the VF driver completes its remove()
+ * callback.
+ *
+ * Locking is achieved by the driver core. A VF driver cannot be probed until
+ * pci_enable_sriov() is called and pci_disable_sriov() does not return until
+ * all VF drivers have completed their remove().
+ *
+ * The PF driver must call pci_disable_sriov() before it begins to destroy the
+ * drvdata.
+ */
+void *pci_iov_get_pf_drvdata(struct pci_dev *dev, struct pci_driver *pf_driver)
+{
+	struct pci_dev *pf_dev;
+
+	if (!dev->is_virtfn)
+		return ERR_PTR(-EINVAL);
+	pf_dev = dev->physfn;
+	if (pf_dev->driver != pf_driver)
+		return ERR_PTR(-EINVAL);
+	return pci_get_drvdata(pf_dev);
+}
+EXPORT_SYMBOL_GPL(pci_iov_get_pf_drvdata);
+
 /*
  * Per SR-IOV spec sec 3.3.10 and 3.3.11, First VF Offset and VF Stride may
  * change when NumVFs changes.
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 2337512e67f0..639a0a239774 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -2154,6 +2154,7 @@ void __iomem *pci_ioremap_wc_bar(struct pci_dev *pdev, int bar);
 int pci_iov_virtfn_bus(struct pci_dev *dev, int id);
 int pci_iov_virtfn_devfn(struct pci_dev *dev, int id);
 int pci_iov_vf_id(struct pci_dev *dev);
+void *pci_iov_get_pf_drvdata(struct pci_dev *dev, struct pci_driver *pf_driver);
 int pci_enable_sriov(struct pci_dev *dev, int nr_virtfn);
 void pci_disable_sriov(struct pci_dev *dev);

@@ -2187,6 +2188,12 @@ static inline int pci_iov_vf_id(struct pci_dev *dev)
 	return -ENOSYS;
 }

+static inline void *pci_iov_get_pf_drvdata(struct pci_dev *dev,
+					   struct pci_driver *pf_driver)
+{
+	return ERR_PTR(-EINVAL);
+}
+
 static inline int pci_enable_sriov(struct pci_dev *dev, int nr_virtfn)
 { return -ENODEV; }

-- 
2.18.1