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Re: [Xen-devel] [PATCH] xenpv: don't BUG when failing to setup NMI callback

On 06/13/2014 08:45 AM, Konrad Rzeszutek Wilk wrote:
On Fri, Jun 13, 2014 at 01:26:28PM +0200, Vitaly Kuznetsov wrote:
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

(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@xxxxxxxxxx>
Oh, we had a similar patch - somebody reported it earlier - and we
just checked the version of Xen:


But I can't remember why I didn't post it.

However I do like your path of checking the 'ret'.

Should we be checking both ret and version? NMI callback registration can return -EINVAL for other reasons (non-canonical address, for example).


Vitaly, could expand your patch to also do a check in cvt_gate_to_trap
so that it won't enable the NMI handler and then lets pick your

  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 */
+       }
  void __init xen_pvmmu_arch_setup(void)

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