Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Jin, Yao
2020-08-10T02:03:24Z

Hi Peter,

On 8/7/2020 5:02 PM, peterz@infradead.org wrote:
> On Fri, Aug 07, 2020 at 02:24:30PM +0800, Jin, Yao wrote:
>> Hi Peter,
>>
>> On 8/6/2020 7:00 PM, peterz@infradead.org wrote:
>>> On Thu, Aug 06, 2020 at 11:18:27AM +0200, peterz@infradead.org wrote:
>>>
>>>> Suppose we have nested virt:
>>>>
>>>> L0-hv
>>>> |
>>>> G0/L1-hv
>>>> |
>>>> G1
>>>>
>>>> And we're running in G0, then:
>>>>
>>>> - 'exclude_hv' would exclude L0 events
>>>> - 'exclude_host' would ... exclude L1-hv events?
>>>> - 'exclude_guest' would ... exclude G1 events?
>>>
>>> So in arch/x86/events/intel/core.c we have:
>>>
>>> static inline void intel_set_masks(struct perf_event *event, int idx)
>>> {
>>> struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>>
>>> if (event->attr.exclude_host)
>>> __set_bit(idx, (unsigned long *)&cpuc->intel_ctrl_guest_mask);
>>> if (event->attr.exclude_guest)
>>> __set_bit(idx, (unsigned long *)&cpuc->intel_ctrl_host_mask);
>>> if (event_is_checkpointed(event))
>>> __set_bit(idx, (unsigned long *)&cpuc->intel_cp_status);
>>> }
>>>
>>
>> exclude_host is now set by guest (pmc_reprogram_counter,
>> arch/x86/kvm/pmu.c). When enabling the event, we can check exclude_host to
>> know if it's a guest.
>>
>> Otherwise we may need more flags in event->attr to indicate the status.
>>
>>> which is, afaict, just plain wrong. Should that not be something like:
>>>
>>> if (!event->attr.exclude_host)
>>> __set_bit(idx, (unsigned long *)&cpuc->intel_ctrl_host_mask);
>>> if (!event->attr.exclude_guest)
>>> __set_bit(idx, (unsigned long *)&cpuc->intel_ctrl_guest_mask);
>>>
>>>
>>
>> How can we know it's guest or host even if exclude_host is set in guest?
>
> I'm not following you, consider:
>
> xh xg h g h' g'
> 0 0 0 0 1 1
> 0 1 1 0 1 0
> 1 0 0 1 0 1
> 1 1 1 1 0 0
>
>

Thanks for the table! It clearly shows the combinations of different conditions.

My understanding is:

xh = exclude_host
xg = exclude_guest
h = intel_ctrl_host_mask (before)
g = intel_ctrl_guest_mask (before)
h' = intel_ctrl_host_mask (after)
g' = intel_ctrl_guest_mask (after)

For guest, exclude_host = 1 and exclude_guest = 0

xh xg h g h' g'
1 0 0 1 0 1

before/after values are not changed.

For host, exclude_host = 0 and exclude_guest = 1

xh xg h g h' g'
0 1 1 0 1 0

before/after values are not changed.

> So the 0,0 and 1,1 cases get flipped. I have a suspicion, but this
> _really_ should have fat comments all over :-(
>

I'm not very sure about other cases.

xh xg h g h' g'
0 0 0 0 1 1
1 1 1 1 0 0

The before/after values are just reversed. I don't know if there will be some negative impacts?
Maybe we need more reviews here.

> What a sodding trainwreck..
>

:(

Thanks
Jin Yao