[Xen-devel] [PATCH v2] x86/xen: Return from panic notifier

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

Currently execution of panic() continues until Xen's panic notifier
(xen_panic_event()) is called at which point we make a hypercall that
never returns.

This means that any notifier that is supposed to be called later as
well as significant part of panic() code (such as pstore writes from
kmsg_dump()) is never executed.

There is no reason for xen_panic_event() to be this last point in
execution since panic()'s emergency_restart() will call into
xen_emergency_restart() from where we can perform our hypercall.

Nevertheless, we will provide xen_legacy_crash boot option that will
preserve original behavior during crash. This option could be used,
for example, if running kernel dumper (which happens after panic
notifiers) is undesirable.

Reported-by: James Dingwall <james@xxxxxxxxxxxxxx>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---

v2: Added xen_legacy_crash boot option to preserve current behaviour. My
earlier suggestion to create an external dumper (for Xen toolstack)
had some corner cases and dealing with them was becoming too much logic
for my taste.


 .../admin-guide/kernel-parameters.txt         |  4 +++
 arch/x86/xen/enlighten.c                      | 28 +++++++++++++++++--
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 4c1971960afa..5ea005c9e2d6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5267,6 +5267,10 @@
                                the unplug protocol
                        never -- do not unplug even if version check succeeds
 
+       xen_legacy_crash        [X86,XEN]
+                       Crash from Xen panic notifier, without executing late
+                       panic() code such as dumping handler.
+
        xen_nopvspin    [X86,XEN]
                        Disables the ticketlock slowpath using Xen PV
                        optimizations.
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 750f46ad018a..205b1176084f 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -269,19 +269,41 @@ void xen_reboot(int reason)
                BUG();
 }
 
+static int reboot_reason = SHUTDOWN_reboot;
+static bool xen_legacy_crash;
 void xen_emergency_restart(void)
 {
-       xen_reboot(SHUTDOWN_reboot);
+       xen_reboot(reboot_reason);
 }
 
 static int
 xen_panic_event(struct notifier_block *this, unsigned long event, void *ptr)
 {
-       if (!kexec_crash_loaded())
-               xen_reboot(SHUTDOWN_crash);
+       if (!kexec_crash_loaded()) {
+               if (xen_legacy_crash)
+                       xen_reboot(SHUTDOWN_crash);
+
+               reboot_reason = SHUTDOWN_crash;
+
+               /*
+                * If panic_timeout==0 then we are supposed to wait forever.
+                * However, to preserve original dom0 behavior we have to drop
+                * into hypervisor. (domU behavior is controlled by its
+                * config file)
+                */
+               if (panic_timeout == 0)
+                       panic_timeout = -1;
+       }
        return NOTIFY_DONE;
 }
 
+static int __init parse_xen_legacy_crash(char *arg)
+{
+       xen_legacy_crash = true;
+       return 0;
+}
+early_param("xen_legacy_crash", parse_xen_legacy_crash);
+
 static struct notifier_block xen_panic_block = {
        .notifier_call = xen_panic_event,
        .priority = INT_MIN
-- 
2.17.1


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