[PATCH v2] 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.

Changes in v2:
- skip nmi in cvt_gate_to_trap() for Xen < 3.2
- do not call xen_enable_nmi() when running under Xen < 3.2
- never BUG() in xen_enable_nmi()

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/xen/enlighten.c |  9 +++++----
 arch/x86/xen/setup.c     | 17 ++++++++++++++---
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index f17b292..60ec5ef 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -746,12 +746,13 @@ static int cvt_gate_to_trap(int vector, const gate_desc *val,
 		 */
 		;
 #endif
-	} else if (addr == (unsigned long)nmi)
+	} else if (addr == (unsigned long)nmi) {
 		/*
-		 * Use the native version as well.
+		 * Use the native version as well but require Xen >= 3.2
 		 */
-		;
-	else {
+		if (!xen_running_on_version_or_later(3, 2))
+			return 0;
+	} else {
 		/* Some other trap using IST? */
 		if (WARN_ON(val->ist != 0))
 			return 0;
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 821a11a..fdc73d3 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -593,8 +593,14 @@ void xen_enable_syscall(void)
 void xen_enable_nmi(void)
 {
 #ifdef CONFIG_X86_64
-	if (register_callback(CALLBACKTYPE_nmi, (char *)nmi))
-		BUG();
+	int ret;
+
+	ret = register_callback(CALLBACKTYPE_nmi, (char *)nmi);
+	if (ret != 0) {
+		/* Hypervisor probably forbids us to register NMI callback or
+		   some other error happened */
+		pr_warn("xen: failed to register NMI callback: %d\n", ret);
+	}
 #endif
 }
 void __init xen_pvmmu_arch_setup(void)
@@ -611,7 +617,12 @@ void __init xen_pvmmu_arch_setup(void)
 
 	xen_enable_sysenter();
 	xen_enable_syscall();
-	xen_enable_nmi();
+
+	/* Xen versions prior to 3.2 forbid DomUs to register NMI callbacks */
+	if (xen_running_on_version_or_later(3, 2))
+		xen_enable_nmi();
+	else
+		pr_warn("xen: skipping NMI callback registration for Xen < 3.2");
 }
 
 /* This function is not called for HVM domains */
-- 
1.9.3