Re: [Xen-devel] [PATCH] x86/vPMU: constrain MSR_IA32_DS_AREA loads

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.12.15 at 16:12, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 12/18/2015 01:21 AM, Tian, Kevin wrote:
>>> From: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
>>> Sent: Thursday, December 17, 2015 10:42 PM
>>>
>>> On 12/17/2015 09:29 AM, Jan Beulich wrote:
>>>>>>> On 17.12.15 at 15:26, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> On 12/17/2015 09:18 AM, Jan Beulich wrote:
>>>>>>>>> On 17.12.15 at 15:12, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>>>> On 12/17/2015 09:01 AM, Jan Beulich wrote:
>>>>>>>> @@ -415,8 +416,10 @@ static int core2_vpmu_verify(struct vcpu
>>>>>>>>                  enabled_cntrs |= (1ULL << i);
>>>>>>>>          }
>>>>>>>>
>>>>>>>> -    if ( vpmu_is_set(vcpu_vpmu(v), VPMU_CPU_HAS_DS) &&
>>>>>>>> -         !is_canonical_address(core2_vpmu_cxt->ds_area) )
>>>>>>>> +    if ( vpmu_is_set(vpmu, VPMU_CPU_HAS_DS) &&
>>>>>>>> +         !(has_hvm_container_vcpu(v)
>>>>>>>> +           ? is_canonical_address(core2_vpmu_cxt->ds_area)
>>>>>>>> +           : __addr_ok(core2_vpmu_cxt->ds_area)) )
>>>>>>> Should we instead of (or in addition to) this also make the same change
>>>>>>> in core2_vpmu_do_wrmsr()?
>>>>>> Currently there's no need for this since - afaict - PV guests can't
>>>>>> write this MSR directly (it's not among the white listed set in
>>>>>> traps.c).
>>>>> Then we probably shouldn't set VPMU_CPU_HAS_DS for PV guests.
>>>> Or add the MSR to the permitted set. You know better than I
>>>> what the best route here is.
>>> I vaguely recall a conversation where we weren't sure whether BTS (which
>>> needs DS area) will work for PV. Something to do with DS address being
>>> in the right context (guest or host). I'd need to find that conversation
>>> (or test BTS on PV).
>>>
>> I guess I don't need to review current patch until you have a conclusion, 
> right? :-)
> 
> All I can say that is BTS does not work on PV (at least as far as perf 
> is concerned, which is the only tool I know that could use it). Which is 
> not surprising given that we can't access DS_AREA MSR.

But in the context of this patch (and what direction a v2 might
need to go) we'd need to assume that MSR can be accessed.

Jan


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