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

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Date:   Mon, 10 Aug 2020 10:03:18 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     peterz@...radead.org
Cc:     mingo@...hat.com, oleg@...hat.com, acme@...nel.org,
        jolsa@...nel.org, Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com,
        alexander.shishkin@...ux.intel.com, mark.rutland@....com
Subject: Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Hi Peter,

On 8/7/2020 5:02 PM, peterz@...radead.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@...radead.org wrote:
>>> On Thu, Aug 06, 2020 at 11:18:27AM +0200, peterz@...radead.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