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

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Message-ID: <aa38dc24-4496-2d26-071e-26e9fbc3dbd4@linux.intel.com>
Date:   Wed, 12 Aug 2020 15:25:03 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>,
        Peter Zijlstra <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

On 2020/8/12 11:52, Jin, Yao wrote:
> Hi Peter,
> 
> On 8/11/2020 4:45 PM, Peter Zijlstra wrote:
>> On Tue, Aug 11, 2020 at 04:31:10PM +0800, Jin, Yao wrote:
>>> Hi Peter,
>>>
>>> On 8/11/2020 3:59 PM, Peter Zijlstra wrote:
>>>> On Tue, Aug 11, 2020 at 03:50:43PM +0800, Jin, Yao wrote:
>>>>> Could I post v2 which basically refers to your patch but removes some
>>>>> conditions since I see some issues in test if we use these conditions.
>>>>>
>>>>>    1. Remove '!event->attr.exclude_hv || !event->attr.exclude_host ||
>>>>>       !event->attr.exclude_guest' at the entry of sanitize_sample_regs().
>>>>>
>>>>>    2. Remove '!attr.exclude_hv || !attr.exclude_host || 
>>>>> !attr.exclude_guest'
>>>>>       at the perf_event_open syscall entry.
>>>>
>>>> exclude_host, maybe -- due to the dodgy semantics of it, but the others
>>>> should definitely be there.
>>>>
>>>
>>> exclude_guest and exclude_hv are tricky too.
>>>
>>> If we do 'perf record -e cycles:u' in both host and guest, we can see:
>>>
>>> event->attr.exclude_guest = 0
>>>
>>> thus sanitize_sample_regs() returns regs directly even if exclude_kernel 
>>> = 1.
>>>
>>> And in guest, exclude_hv = 0, it's out of my expectation too.
>>
>> I'm confused, how can 'perf record -e cycles:u' _ever_ have
>> exclude_guest=0, exclude_hv=0 ? That simply makes no sense and is utterly
>> broken.
>>
>> You explicitly ask for userspace-only, reporting hypervisor or guest
>> events is a straight up bug.

Guest events = guest user-space events + guest kernel-space events.

Some perf users on the host may only want to count guest user space events.

>>
> 
> If we run 'perf record -e cycles:u',
> 
> 1. On host, exclude_guest = 0 and exclude_hv = 1
> 
> perf tool doesn't specially set 'exclude_guest' when it parses the 'u' 
> modifier. I agree that can be improved. I will post a perf tool patch to 
> fix that.
> 
> 2. On guest, exclude_guest = 0 and exclude_hv = 0.
> 
> For exclude_hv = 0, it looks like a bug but x86 doesn't use exclude_hv. But 
> yes, we should fix that.
> 
> CC Like Xu <like.xu@...ux.intel.com>.
> 
> Thanks
> Jin Yao