[PATCH] xenpv: don't BUG when failing to setup NMI callback

[Vitaly Kuznetsov <vkuznets@redhat.com>]

some old Xen hypervisors (prior to 3.2) forbid DomUs to register
NMI callbacks. E.g. we have the following code in xen-3.1:

    if ( (d->domain_id != 0) || (v->vcpu_id != 0) )
        return -EINVAL;

Commit 6efa20e49b9cb1db1ab66870cc37323474a75a13 introduced kernel
crash in case PV guest fails to register NMI callback. All x86_64
PV guests will fail to boot on top of such hypervisors (RHEL5
example):

(XEN) traps.c:405:d7 Unhandled invalid opcode fault/trap [#6] in domain 7 on VCPU 0 [ec=0000]
(XEN) domain_crash_sync called from entry.S
(XEN) Domain 7 (vcpu#0) crashed on cpu#3:
(XEN) ----[ Xen-3.1.2-389.el5  x86_64  debug=n  Not tainted ]----
(XEN) CPU:    3
(XEN) RIP:    e033:[<ffffffff81004d96>]
(XEN) RFLAGS: 0000000000000282   CONTEXT: guest
(XEN) rax: ffffffffffffffea   rbx: 0000000000000000   rcx: 0000000000000002
(XEN) rdx: 0000000000000001   rsi: ffffffff81b0fe28   rdi: 0000000000000000
(XEN) rbp: ffffffff81b0fe40   rsp: ffffffff81b0fde8   r8:  0000000000000000
(XEN) r9:  ffffffff81b0fdd0   r10: 0000000000007ff0   r11: 00000000ffffffff
(XEN) r12: ffffffff81d65900   r13: 0000000000000000   r14: 0000000000000000
(XEN) r15: 0000000000000000   cr0: 0000000080050033   cr4: 00000000000026b0
(XEN) cr3: 000000013a263000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
...

However it is possible to proceed without NMI callback registered.
Change BUG() with warning in case of -EINVAL.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/xen/setup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 821a11a..5b8b180 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -593,8 +593,17 @@ void xen_enable_syscall(void)
 void xen_enable_nmi(void)
 {
 #ifdef CONFIG_X86_64
-	if (register_callback(CALLBACKTYPE_nmi, (char *)nmi))
+	int ret;
+
+	ret = register_callback(CALLBACKTYPE_nmi, (char *)nmi);
+	if (ret == -EINVAL) {
+		/* Hypervisor probably forbids us to register NMI callback,
+		   that is expected when running on top of Xen-3.1 and older */
+		pr_warn("xen: failed to register NMI callback\n");
+	} else if (ret != 0) {
+		/* Other hypervisor failure */
 		BUG();
+	}
 #endif
 }
 void __init xen_pvmmu_arch_setup(void)
-- 
1.9.3