PCI SERR NMI may cause Xen to deadlock

[David Vrabel <david.vrabel@citrix.com>]

If a PCI System Error (SERR) is asserted it causes an NMI. If this NMI
occurs while the CPU is in printk() then Xen may deadlock as
pci_serr_error() calls console_force_unlock() which screws up the
console lock.

I don't think removing the console_force_unlock() is sufficient as it
looks like printk() is unsafe to call from NMI context.  We would like
keep the diagnostic.  Does the following (untested) patch to defer the
printk() to a softirq look like a valid approach?

I also considered passing the NMI to dom0 (like other NMIs are) but I
couldn't see how NMIs were handled in the current upstream pv-ops kernels.

diff -r e2722b24dc09 xen/arch/x86/traps.c
--- a/xen/arch/x86/traps.c	Thu Jan 26 17:43:31 2012 +0000
+++ b/xen/arch/x86/traps.c	Tue Jan 31 16:28:45 2012 +0000
@@ -3173,6 +3173,11 @@ static void nmi_mce_softirq(void)
     st->vcpu = NULL;
 }

+static void pci_serr_softirq(void)
+{
+    printk("\n\nNMI - PCI system error (SERR)\n");
+}
+
 void async_exception_cleanup(struct vcpu *curr)
 {
     int trap;
@@ -3259,10 +3264,11 @@ static void nmi_dom0_report(unsigned int

 static void pci_serr_error(struct cpu_user_regs *regs)
 {
-    console_force_unlock();
-    printk("\n\nNMI - PCI system error (SERR)\n");
-
     outb((inb(0x61) & 0x0f) | 0x04, 0x61); /* clear-and-disable the PCI
SERR error line. */
+
+    /* Would like to print a diagnostic here but can't call printk()
+       from NMI context -- raise a softirq instead. */
+    raise_softirq(PCI_SERR_SOFTIRQ);
 }

 static void io_check_error(struct cpu_user_regs *regs)
@@ -3563,6 +3569,7 @@ void __init trap_init(void)
     cpu_init();

     open_softirq(NMI_MCE_SOFTIRQ, nmi_mce_softirq);
+    open_softirq(PCI_SERR_SOFTIRQ, pci_serr_softirq);
 }

 long register_guest_nmi_callback(unsigned long address)
diff -r e2722b24dc09 xen/include/asm-x86/softirq.h
--- a/xen/include/asm-x86/softirq.h	Thu Jan 26 17:43:31 2012 +0000
+++ b/xen/include/asm-x86/softirq.h	Tue Jan 31 16:28:45 2012 +0000
@@ -6,6 +6,7 @@
 #define VCPU_KICK_SOFTIRQ      (NR_COMMON_SOFTIRQS + 2)

 #define MACHINE_CHECK_SOFTIRQ  (NR_COMMON_SOFTIRQS + 3)
-#define NR_ARCH_SOFTIRQS       4
+#define PCI_SERR_SOFTIRQ       (NR_COMMON_SOFTIRQS + 4)
+#define NR_ARCH_SOFTIRQS       5

 #endif /* __ASM_SOFTIRQ_H__ */

David