Re: [Xen-devel] [PATCH RFC 3/9] xen: Force-enable relevant MSR events; optimize the number of sent MSR events

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.07.14 at 10:02, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 07/02/2014 06:43 PM, Jan Beulich wrote:
>>>>> On 02.07.14 at 17:35, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 02/07/14 14:33, Razvan Cojocaru wrote:
>>>> @@ -700,6 +700,25 @@ void vmx_disable_intercept_for_msr(struct vcpu *v, 
>>>> u32 
> msr, int type)
>>>>      if ( msr_bitmap == NULL )
>>>>          return;
>>>>  
>>>> +    /* Filter out MSR-s needed by the memory introspection engine */
>>>> +    switch ( msr )
>>>> +    {
>>>> +    case MSR_IA32_SYSENTER_EIP:
>>>> +    case MSR_IA32_SYSENTER_ESP:
>>>> +    case MSR_IA32_SYSENTER_CS:
>>>> +    case MSR_IA32_MC0_CTL:
>>>> +    case MSR_STAR:
>>>> +    case MSR_LSTAR:
>>>> +
>>>
>>> Given the performance implications of forcing interception of these
>>> MSRs, it would be gated on mem_access being active for the domain.
>> 
>> Absolutely.
> 
> Unfortunately the call to vmx_disable_intercept_for_msr() happens _very_
> early, and by the time our application gets to enable mem_access on the
> domain, the interception for these MSRs has already been disabled, with
> unacceptable consequences.
> 
> I've tested this with an "if (
> mem_event_check_ring(&d->mem_event->access) )" test.
> 
> Also, ideally we'd like to be able to start monitoring an already
> started domain, and in that case the mem_access test would be useless
> even considering a workaround for the case above.

All understood, but not penalizing non-monitored VMs has certainly
higher priority.

Jan


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