[PATCH RFC] pci: expose function reset capability in sysfs

["Michael S. Tsirkin" <mst@redhat.com>]

Some devices allow an individual function to be reset without affecting
other functions in the same device: that's what pci_reset_function does.
For devices that have this support, expose reset attribite in sysfs.

This is useful e.g. for virtualization, where a qemu userspace
process wants to reset the device when the guest is started/reset,
to emulate machine reboot as closely as possible.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Jesse, all,
could you please comment on whether the following approach looks sane?
Compile-tested only at this point.  I'm also not sure whether the
CAP_SYS_ADMIN check is necessary: maybe 400 permissions on the sysfs
file are sufficient?

 drivers/pci/pci-sysfs.c  |   37 +++++++++++++++++++++++++++++++++++++
 drivers/pci/pci.c        |   16 ++++++++++++++++
 drivers/pci/pci.h        |    1 +
 include/linux/kvm_host.h |    1 +
 virt/kvm/irq_comm.c      |    4 +++-
 5 files changed, 58 insertions(+), 1 deletions(-)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 85ebd02..92805e8 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -916,6 +916,28 @@ int __attribute__ ((weak)) pcibios_add_platform_entries(struct pci_dev *dev)
 	return 0;
 }
 
+static ssize_t reset_store(struct device *dev,
+			   struct device_attribute *attr, const char *buf,
+			   size_t count)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+	unsigned long val;
+	ssize_t result = strict_strtoul(buf, 0, &val);
+
+	if (result < 0)
+		return result;
+
+	/* this can crash the machine when done on the "wrong" device */
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (val != 1)
+		return -EINVAL;
+	return pci_reset_function(pdev);
+}
+
+static struct device_attribute reset_attr = __ATTR(reset, 0200, NULL, reset_store);
+
 static int pci_create_capabilities_sysfs(struct pci_dev *dev)
 {
 	int retval;
@@ -943,7 +965,21 @@ static int pci_create_capabilities_sysfs(struct pci_dev *dev)
 	/* Active State Power Management */
 	pcie_aspm_create_sysfs_dev_files(dev);
 
+	if (!pci_probe_reset_function(dev)) {
+		retval = device_create_file(&dev->dev, &reset_attr);
+		if (retval)
+			goto error;
+	}
 	return 0;
+
+error:
+	pcie_aspm_remove_sysfs_dev_files(dev);
+	if (dev->vpd && dev->vpd->attr) {
+		sysfs_remove_bin_file(&dev->dev.kobj, dev->vpd->attr);
+		kfree(dev->vpd->attr);
+	}
+
+	return retval;
 }
 
 int __must_check pci_create_sysfs_dev_files (struct pci_dev *pdev)
@@ -1037,6 +1073,7 @@ static void pci_remove_capabilities_sysfs(struct pci_dev *dev)
 	}
 
 	pcie_aspm_remove_sysfs_dev_files(dev);
+	device_remove_file(&dev->dev, &reset_attr);
 }
 
 /**
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index dbd0f94..f6d1c6c 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2260,6 +2260,22 @@ int __pci_reset_function(struct pci_dev *dev)
 EXPORT_SYMBOL_GPL(__pci_reset_function);
 
 /**
+ * pci_probe_reset_function - check whether the device can be safely reset
+ * @dev: PCI device to reset
+ *
+ * Some devices allow an individual function to be reset without affecting
+ * other functions in the same device.  The PCI device must be responsive
+ * to PCI config space in order to use this function.
+ *
+ * Returns 0 if the device function can be reset or negative if the
+ * device doesn't support resetting a single function.
+ */
+int pci_probe_reset_function(struct pci_dev *dev)
+{
+	return pci_dev_reset(dev, 1);
+}
+
+/**
  * pci_reset_function - quiesce and reset a PCI device function
  * @dev: PCI device to reset
  *
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index f73bcbe..60a3811 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -16,6 +16,7 @@ extern void pci_cleanup_rom(struct pci_dev *dev);
 extern int pci_mmap_fits(struct pci_dev *pdev, int resno,
 			 struct vm_area_struct *vma);
 #endif
+int pci_probe_reset_function(struct pci_dev *dev);
 
 /**
  * struct pci_platform_pm_ops - Firmware PM callbacks
-- 
1.6.2.5