Re: [PATCH 08/11] perf tool: precise mode requires exclude_guest

[David Ahern <dsahern@gmail.com>]

In an attempt to jump start this thread...

On 8/3/12 7:51 AM, Robert Richter wrote:
> On 26.07.12 10:08:29, Peter Zijlstra wrote:
>> On Wed, 2012-07-25 at 23:16 -0600, David Ahern wrote:
>>
>>> Peter's patch (see https://lkml.org/lkml/2012/7/9/298) changes kernel
>>> side to require the use of exclude_guest if the precise modifier is
>>> used, returning -EOPNOTSUPP if exclude_guest is not set. This patch goes
>>> after the user experience: Today if a user specifies -e <event>:p all
>>> other modifiers are reset - including exclude_guest. Going forward we
>>> need :p to imply :pH if a user has not specified a GH modifer.
>>>
>>> We could do nothing and handle the unsupported error and try setting the
>>> exclude_guest option - like perf handles other new parameters. But
>>> EOPNOTSUPP is not uniquely tied to this error -- e.g., it could be the
>>> BTS is not supported (:pp). Also, we have no easy way to discriminate :p
>>> from :pG or :pGH. It seems to me perf should not silently undo a user
>>> request on the modifier, but inform the user the request is wrong. For
>>> example if a user request -e cycles:pG it should not be silently turned
>>> into :pH.
>>>
>>> And then yesterday, Robert stated that none of the exclude_xxxx
>>> modifiers can be set for the AMD if the precise modifier is used, so we
>>> cannot blindly set exclude_guest if precise_ip is set.
>>>
>>> So, seems to me perf need's one action for Intel processors and another
>>> for AMD.
>>
>> No, we just need to teach the IBS code about SVM enter/exit.
>
> I aggree that this could be emulated in software by enabling/disabling
> the event with a guest/host switch. And, even better, we add this for
> every pmu in a generic way. E.g. northbridge counter and I guess also
> Intel uncore events do not support G/H counting in hardware. Same to
> other pmus that could be imaginable in the future like counters for
> IOMMUs or other hardware devices.
>
> But, as some pmus are not related to virtualization or other features
> they simply do not need to support those attributes, or we want other
> defaults, e.g. enable it system wide. Detecting features with syscall
> error checking and then falling back to other defaults does not seem
> the right approach to me, because it may require several syscalls to
> check *combinations* of supported attributes, makes error logging and
> detection more difficult due to noisy log messages and because there
> is no strict attribute flag checking in current and older kernels.
>
> I better would like to see a pmu feature flag in the same style as
> with /proc/cpuinfo, e.g.:
>
>   $ cat /sys/bus/event_source/devices/cpu/flags
>   exclude_host exclude_guest
>
> We also need stricter attribute flag checking, esp. of reseved flags
> and for unsupported features in some pmus (I already work on some
> patches for this). Userland then checks flags and sets up syscalls
> according to the reported flags. The goal should be to avoid syscall
> errors at all. Thus, we are able to improve dmesg logging in case of
> errors, currently we do not see any message if a syscall fails.
>
> And finally, if a feature could be emulated, we could provide this
> emulation of an attr flag to all pmus.
>
> Does this make sense?

We need to require exclude_guest when using precise attribute with perf 
else all running VMs on Intel-based servers will crash. I do not have an 
AMD based server to even attempt the preferred solution. Best I can do 
is to attempt to keep this thread alive until someone with one can 
tackle the problem.

David