linux-kernel - [PATCH] xen-pcifront: Handle missed Connected state

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Message-Id: <20220829151536.8578-1-jandryuk@gmail.com>
Date:   Mon, 29 Aug 2022 11:15:36 -0400
From:   Jason Andryuk <jandryuk@...il.com>
To:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Jason Andryuk <jandryuk@...il.com>, xen-devel@...ts.xenproject.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] xen-pcifront: Handle missed Connected state

An HVM guest with linux stubdom and 2 PCI devices failed to start as
libxl timed out waiting for the PCI devices to be added.  It happens
intermittently but with some regularity.  libxl wrote the two xenstore
entries for the devices, but then timed out waiting for backend state 4
(Connected) - the state stayed at 7 (Reconfiguring).  (PCI passthrough
to an HVM with stubdomain is PV passthrough to the stubdomain and then
HVM passthrough with the QEMU inside the stubdomain.)

The stubdom kernel never printed "pcifront pci-0: Installing PCI
frontend", so it seems to have missed state 4 which would have
called pcifront_try_connect -> pcifront_connect_and_init_dma

Have pcifront_detach_devices special-case state Initialised and call
pcifront_connect_and_init_dma.  Don't use pcifront_try_connect because
that sets the xenbus state which may throw off the backend.  After
connecting, skip the remainder of detach_devices since none have been
initialized yet.  When the backend switches to Reconfigured,
pcifront_attach_devices will pick them up again.

Signed-off-by: Jason Andryuk <jandryuk@...il.com>
---
 drivers/pci/xen-pcifront.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

--- a/drivers/pci/xen-pcifront.c
+++ b/drivers/pci/xen-pcifront.c
@@ -1012,13 +1012,26 @@ static int pcifront_detach_devices(struc
 {
 	int err = 0;
 	int i, num_devs;
+	enum xenbus_state state;
 	unsigned int domain, bus, slot, func;
 	struct pci_dev *pci_dev;
 	char str[64];
 
-	if (xenbus_read_driver_state(pdev->xdev->nodename) !=
-	    XenbusStateConnected)
+	state = xenbus_read_driver_state(pdev->xdev->nodename);
+	if (state == XenbusStateInitialised) {
+		dev_dbg(&pdev->xdev->dev, "Handle skipped connect.\n");
+		/* We missed Connected and need to initialize. */
+		err = pcifront_connect_and_init_dma(pdev);
+		if (err && err != -EEXIST) {
+			xenbus_dev_fatal(pdev->xdev, err,
+					 "Error setting up PCI Frontend");
+			goto out;
+		}
+
+		goto out_switch_state;
+	} else if (state != XenbusStateConnected) {
 		goto out;
+	}
 
 	err = xenbus_scanf(XBT_NIL, pdev->xdev->otherend, "num_devs", "%d",
 			   &num_devs);
@@ -1079,6 +1092,7 @@ static int pcifront_detach_devices(struc
 			domain, bus, slot, func);
 	}
 
+ out_switch_state:
 	err = xenbus_switch_state(pdev->xdev, XenbusStateReconfiguring);
 
 out: