Re: Intermittent fatal page fault with XEN 4.3.1 (Centos 6.3 DOM0 with linux kernel 3.10.16.)

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 07/11/13 16:53, Jan Beulich wrote:
>>>> On 07.11.13 at 17:02, <Jeff_Zimmerman@McAfee.com> wrote:
>> On Nov 7, 2013, at 7:57 AM, Jan Beulich <JBeulich@suse.com>
>>  wrote:
>>
>>>>>> On 07.11.13 at 16:41, <Jeff_Zimmerman@McAfee.com> wrote:
>>>> On Nov 7, 2013, at 1:30 AM, Ian Campbell <Ian.Campbell@citrix.com>  wrote:
>>>>> I was also wondering about the behaviour of using vmx instructions in a
>>>>> guest despite vmx not being visible in cpuid...
>>>>>
>>>> We have found in our situation this is exactly the case. To verify we wrote 
>>>> some
>>>> test code that makes vmx calls without checking cupid. On bare hardware the 
>>>> program
>>>> executes as expected. In a VM on Xen it causes the hypervisor to panic.
>>> You trying it doesn't yet imply that Windows also does so.
>>>
>>> Also, you say "program" - are you using these from user mode code?
>> Yes, from windows run as a privileged user. Windows XP sp3 can cause the 
>> crash.
>> It seems windows 7 has better security, we cannot crash the system from a 
>> win7 guest.
> Which is sort of odd. Anyway - care to try the attached patch?
>
> Jan
>

I have managed to reproduce the issue, and the patch appears to fix things.

I have to admit to being very surprised that the VMX hardware doesn't
check CR4.VMXE before causing a vmexit.

Reviewed-and-tested-by: Andrew Cooper <andrew.cooper3@citrix.com>