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

[Konrad Rzeszutek Wilk]

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
> 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@xxxxxxxxxx>

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

http://lists.xenproject.org/archives/html/xen-devel/2014-05/msg01474.html

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

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

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
patch?

> ---
>  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
> 
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