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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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.


Thanks,
Razvan Cojocaru

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