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 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.

>> +    {
>> +    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:
>> +
>> +        gdprintk(XENLOG_DEBUG, "MSR 0x%08x needed for "
>> +            "memory introspection, still intercepted\n", msr);
> 
> I you are going to split this line then do it at the %08x so the string
> is still grepable.
> 
> How often do these messages trigger?  It seems like it could be
> contribute to a lot of logspam.

As far as I've seen, never more than a handful of them (maybe 5-6 lines)
when the domain starts, never to be seen again. Definitely not log spam
material.


Thanks,
Razvan Cojocaru

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