Re: [Xen-devel] [PATCH V3 2/2] Xen/PCIback: Implement PCI flr/slot/bus reset with 'reset' SysFS attribute

[Govinda Tatti <Govinda.Tatti@xxxxxxxxxx>]

Thanks Jan for your review comments. Please see below for my comments.

On 12/8/2017 3:34 AM, Jan Beulich wrote:

On 07.12.17 at 23:21, <Govinda.Tatti@xxxxxxxxxx> wrote:

Due to the complexity with the PCI lock we cannot do the reset when a
device is bound ('echo $BDF > bind') or when unbound ('echo $BDF > unbind')
as the pci_[slot|bus]_reset also takes the same lock resulting in a
dead-lock.

It took me a moment to figure that here you're referring to the
process of (un)binding, not the state. To avoid that ambiguity in
wording, how about "... we cannot do the reset while a device is
being bound (...) or while it is being unbound ..."?

Sure, I will fix it.

--- a/Documentation/ABI/testing/sysfs-driver-pciback
+++ b/Documentation/ABI/testing/sysfs-driver-pciback
@@ -11,3 +11,18 @@ Description:
                 #echo 00:19.0-E0:2:FF > /sys/bus/pci/drivers/pciback/quirks
                 will allow the guest to read and write to the configuration
                 register 0x0E.
+
+What:           /sys/bus/pci/drivers/pciback/reset
+Date:           Dec 2017
+KernelVersion:  4.15
+Contact:        xen-devel@xxxxxxxxxxxxxxxxxxxx 
+Description:
+                An option to perform a flr/slot/bus reset when a PCI device
+		is owned by Xen PCI backend. Writing a string of DDDD:BB:DD.F

SSSS:BB:DD.F (or else the D-s are ambiguous, the more that "domain"
in Xen code is ambiguous anyway - I continue to be mislead by struct
pcistub_device_id's domain field)

Thanks for catching this issue. I will fix it.

Also I assume the SSSS part is optional (default zero), which
probably can and should be expressed in some way.

SSSS can be 0 or non-zero, subject to system configuration.

--- a/drivers/xen/xen-pciback/pci_stub.c
+++ b/drivers/xen/xen-pciback/pci_stub.c
@@ -313,6 +313,102 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
 	up_write(&pcistub_sem);
 }
 
+struct pcistub_args {
+	const struct pci_dev *dev;
+	unsigned int dcount;

The sole use of this field is for a debug message. Why not drop it
and make "dev" the "data" argument without further indirection?

I prefer to keep this data structure since it will be helpful to debug any issues
or for future enhancements.

+static int pcistub_device_search(struct pci_dev *dev, void *data)
+{
+	struct pcistub_device *psdev;
+	struct pcistub_args *arg = data;
+	bool found = false;
+	unsigned long flags;
+
+	spin_lock_irqsave(&pcistub_devices_lock, flags);
+
+	list_for_each_entry(psdev, &pcistub_devices, dev_list) {
+		if (psdev->dev == dev) {
+			found = true;
+			arg->dcount++;
+			break;

Neither here nor in the caller I can see a check whether the device
is currently assigned to a guest. Ownership by pciback alone imo is
not sufficient to allow a reset to be performed.

I can add the following check

if ((psdev->dev == dev) && (pci_is_dev_assigned(dev)))

+static int pcistub_device_reset(struct pci_dev *dev)
+{
+	struct xen_pcibk_dev_data *dev_data;
+	bool slot = false, bus = false;
+	struct pcistub_args arg = {};
+
+	if (!dev)
+		return -EINVAL;
+
+	dev_dbg(&dev->dev, "[%s]\n", __func__);
+
+	/* First check and try FLR */
+	if (pcie_has_flr(dev)) {
+		dev_dbg(&dev->dev, "resetting %s device using FLR\n",
+			pci_name(dev));
+		pcie_flr(dev);

The lack of error check here puzzled me, but I see the function
indeed returns void right now. I think the prereq patch should
change this along with exporting the function - you really don't
want the device to be handed to a guest when the FLR timed
out.

We will change pcie_flr() to return error code. I will make this change
in the next version of this patch.

+		return 0;
+	}
+
+	if (!pci_probe_reset_slot(dev->slot))
+		slot = true;
+	else if ((!pci_probe_reset_bus(dev->bus)) &&
+		 (!pci_is_root_bus(dev->bus)))

Too many parentheses for my taste.

I will fix it.

+static ssize_t reset_store(struct device_driver *drv, const char *buf,
+			   size_t count)
+{
+	struct pcistub_device *psdev;
+	int domain, bus, slot, func;
+	int err;
+
+	err = str_to_slot(buf, &domain, &bus, &slot, &func);
+	if (err)
+		return err;
+
+	psdev = pcistub_device_find(domain, bus, slot, func);
+	if (psdev) {
+		err = pcistub_device_reset(psdev->dev);
+		pcistub_device_put(psdev);
+	} else {
+		err = -ENODEV;
+	}
+
+	if (!err)
+		err = count;
+
+	return err;
+}
+static DRIVER_ATTR_WO(reset);

Would it be worth for reads of the file to return whether the device
can be reset this way (i.e. the result of the checks you do before
actually doing the reset)?

I don't think so. Plus, it makes this interface and its usage more complicated.

Cheers
GOVINDA

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