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Subject: Re: [RFC PATCH] perf/core: don't sample kernel regs upon skid
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Jin Yao <yao.jin@linux.intel.com>
References: <20180702151250.14536-1-mark.rutland@arm.com>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <98e69f42-bae4-d2a9-727b-1eaee91fe3c0@oracle.com>
Date:   Wed, 11 Jul 2018 11:33:35 -0400
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 07/11/2018 01:59 AM, Mark Rutland wrote:
> On Mon, Jul 09, 2018 at 06:42:29PM -0400, Boris Ostrovsky wrote:
>> On 07/02/2018 12:02 PM, Mark Rutland wrote:
>>> On Mon, Jul 02, 2018 at 05:46:55PM +0200, Peter Zijlstra wrote:
>>>> On Mon, Jul 02, 2018 at 04:12:50PM +0100, Mark Rutland wrote:
>>>>> +static struct pt_regs *perf_get_sample_regs(struct perf_event *event,
>>>>> +					    struct pt_regs *regs)
>>>>> +{
>>>>> +	/*
>>>>> +	 * Due to interrupt latency (AKA "skid"), we may enter the kernel
>>>>> +	 * before taking an overflow, even if the PMU is only counting user
>>>>> +	 * events.
>>>>> +	 *
>>>>> +	 * If we're not counting kernel events, always use the user regs when
>>>>> +	 * sampling.
>>>>> +	 *
>>>>> +	 * TODO: what do we do about sampling a guest's registers? The IP is
>>>>> +	 * special-cased, but for the rest of the regs they'll get the
>>>>> +	 * user/kernel regs depending on whether exclude_kernel is set, which
>>>>> +	 * is nonsensical.
>>>>> +	 *
>>>>> +	 * We can't get at the full set of regs in all cases (e.g. Xen's PV PMU
>>>>> +	 * can't provide the GPRs), so should we just zero the GPRs when in a
>>>>> +	 * guest? Or skip outputting the regs in perf_output_sample?
>>>> Seems daft Xen cannot provide registers; why is that? Boris?
>>> The xen_pmu_regs structure simply doesn't have them, so I assume there's
>>> no API to get them.
>>>
>>> Given we don't currently sample the guest regs, I'd be tempted to just
>>> zero them for now, or skip the sample at output time (if that doesn't
>>> break some other case).
>> (Was out on vacation, couldn't respond earlier)
>>
>> Yes, PV guests only get a limited set of registers passed to the handler
>> by the hypervisor. GPRs are not part of this set.
> Is that also true for Dom0?


Yes. As far as VPMU is concerned there is (almost) no differences
between privileged (i.e. dom0) and unprivileged PV guests.


-boris