Re: [Xen-devel] [PATCH RFC V2 3/6] xen: Force-enable relevant MSR events; optimize the number of sent MSR events

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 07/11/2014 09:19 PM, Andrew Cooper wrote:
> On 11/07/14 19:09, Razvan Cojocaru wrote:
>> On 07/11/2014 08:03 PM, Andrew Cooper wrote:
>>> On 11/07/14 16:43, Razvan Cojocaru wrote:
>>>> Vmx_disable_intercept_for_msr() will now refuse to disable interception of
>>>> MSRs needed for memory introspection. It is not possible to gate this on
>>>> mem_access being active for the domain, since by the time mem_access does
>>>> become active the interception for the interesting MSRs has already been
>>>> disabled (vmx_disable_intercept_for_msr() runs very early on).
>>>>
>>>> Changes since V1:
>>>>  - Replaced printk() with gdprintk(XENLOG_DEBUG, ...).
>>>>
>>>> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>> ---
>>>>  xen/arch/x86/hvm/vmx/vmcs.c |   18 ++++++++++++++++++
>>>>  1 file changed, 18 insertions(+)
>>>>
>>>> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
>>>> index 8ffc562..35fcfcc 100644
>>>> --- a/xen/arch/x86/hvm/vmx/vmcs.c
>>>> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
>>>> @@ -700,6 +700,24 @@ void vmx_disable_intercept_for_msr(struct vcpu *v, 
>>>> u32 msr, int type)
>>>>      if ( msr_bitmap == NULL )
>>>>          return;
>>>>  
>>>> +    /* Filter out MSR-s needed for memory introspection */
>>>> +    switch ( msr )
>>> This absolutely must be gated on mem_events being enabled for the domain.
>>>
>>> It is too much of a performance penalty to apply to domains which are
>>> not being introspected.
>> I understand, but it really runs very early on, and the mem_event part
>> comes in after the MSR interception is disabled. This effectively
>> renders a lot of memory introspection functionality useless.
> 
> In which case the hypercall which enables mem_event needs to prod the
> vmcs state an explicitly enable intercepts for these MSRs. (and
> conversly re-disables intercepts if mem_events are disabled)
> 
> You can probably get away with hvm_funcs to enable and disable mem events.

That makes sense. Would Aravindh's solution also be acceptable to you
(using a Xen command line parameter and gate it based on that rather
than a mem_event listener being present)?


Thanks,
Razvan Cojocaru

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