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From: Jan Beulich <jbeulich@suse.com>
To: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Juergen Gross <jgross@suse.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Konrad Wilk <konrad.wilk@oracle.com>
Subject: [PATCH 2/3] xen-pciback: reconfigure also from backend watch handler
Date: Wed, 7 Apr 2021 16:37:31 +0200	[thread overview]
Message-ID: <74955b48-80b1-3436-06b4-d8569260aa65@suse.com> (raw)
In-Reply-To: <d2ac85d9-0ad1-71a3-fa3b-b99340b3204d@suse.com>

When multiple PCI devices get assigned to a guest right at boot, libxl
incrementally populates the backend tree. The writes for the first of
the devices trigger the backend watch. In turn xen_pcibk_setup_backend()
will set the XenBus state to Initialised, at which point no further
reconfigures would happen unless a device got hotplugged. Arrange for
reconfigure to also get triggered from the backend watch handler.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
---
I will admit that this isn't entirely race-free (with the guest actually
attaching in parallel), but from the looks of it such a race ought to be
benign (not the least thanks to the mutex). Ideally the tool stack
wouldn't write num_devs until all devices had their information
populated. I tried doing so in libxl, but xen_pcibk_setup_backend()
calling xenbus_dev_fatal() when not being able to read that node
prohibits such an approach (or else libxl and driver changes would need
to go into use in lock-step).

I wonder why what is being watched isn't just the num_devs node. Right
now the watch triggers quite frequently without anything relevant
actually having changed (I suppose in at least some cases in response
to writes by the backend itself).

--- a/drivers/xen/xen-pciback/xenbus.c
+++ b/drivers/xen/xen-pciback/xenbus.c
@@ -359,7 +359,8 @@ out:
 	return err;
 }
 
-static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev)
+static int xen_pcibk_reconfigure(struct xen_pcibk_device *pdev,
+				 enum xenbus_state state)
 {
 	int err = 0;
 	int num_devs;
@@ -374,8 +375,7 @@ static int xen_pcibk_reconfigure(struct
 
 	mutex_lock(&pdev->dev_lock);
 	/* Make sure we only reconfigure once */
-	if (xenbus_read_driver_state(pdev->xdev->nodename) !=
-	    XenbusStateReconfiguring)
+	if (xenbus_read_driver_state(pdev->xdev->nodename) != state)
 		goto out;
 
 	err = xenbus_scanf(XBT_NIL, pdev->xdev->nodename, "num_devs", "%d",
@@ -500,6 +500,9 @@ static int xen_pcibk_reconfigure(struct
 		}
 	}
 
+	if (state != XenbusStateReconfiguring)
+		goto out;
+
 	err = xenbus_switch_state(pdev->xdev, XenbusStateReconfigured);
 	if (err) {
 		xenbus_dev_fatal(pdev->xdev, err,
@@ -525,7 +528,7 @@ static void xen_pcibk_frontend_changed(s
 		break;
 
 	case XenbusStateReconfiguring:
-		xen_pcibk_reconfigure(pdev);
+		xen_pcibk_reconfigure(pdev, XenbusStateReconfiguring);
 		break;
 
 	case XenbusStateConnected:
@@ -664,6 +667,10 @@ static void xen_pcibk_be_watch(struct xe
 		xen_pcibk_setup_backend(pdev);
 		break;
 
+	case XenbusStateInitialised:
+		xen_pcibk_reconfigure(pdev, XenbusStateInitialised);
+		break;
+
 	default:
 		break;
 	}



  parent reply	other threads:[~2021-04-07 14:37 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-07 14:35 [PATCH 0/3] xen-pciback: a fix, a workaround, and some simplification Jan Beulich
2021-04-07 14:37 ` [PATCH 1/3] xen-pciback: redo VF placement in the virtual topology Jan Beulich
2021-04-08 22:28   ` Boris Ostrovsky
2021-04-09  8:16     ` Jan Beulich
2021-04-07 14:37 ` Jan Beulich [this message]
2021-04-09 21:43   ` [PATCH 2/3] xen-pciback: reconfigure also from backend watch handler Boris Ostrovsky
2021-04-12  9:44     ` Jan Beulich
2021-04-12 15:55       ` Boris Ostrovsky
2021-04-07 14:37 ` [PATCH 3/3] xen-pciback: simplify vpci's find hook Jan Beulich
2021-04-09 21:45   ` Boris Ostrovsky
2021-04-12  9:50     ` Jan Beulich
2021-04-12 16:05       ` Boris Ostrovsky
2021-04-13  8:09         ` Jan Beulich
2021-04-13 12:54           ` Boris Ostrovsky
2021-04-23  8:05   ` Juergen Gross

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