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SubjectRe: [PATCH] Enhance perf to collect KVM guest os statistics from host side

* Frank Ch. Eigler <fche@redhat.com> wrote:

>
> Ingo Molnar <mingo@elte.hu> writes:
>
> > [...]
> >> >I.e. we really want to be able users to:
> >> >
> >> > 1) have it all working with a single guest, without having to specify 'which'
> >> > guest (qemu PID) to work with. That is the dominant usecase both for
> >> > developers and for a fair portion of testers.
> >>
> >> That's reasonable if we can get it working simply.
> >
> > IMO such ease of use is reasonable and required, full stop.
> > If it cannot be gotten simply then that's a bug: either in the code, or in the
> > design, or in the development process that led to the design. Bugs need
> > fixing. [...]
>
> Perhaps the fact that kvm happens to deal with an interesting application
> area (virtualization) is misleading here. As far as the host kernel or
> other host userspace is concerned, qemu is just some random unprivileged
> userspace program (with some *optional* /dev/kvm services that might happen
> to require temporary root).
>
> As such, perf trying to instrument qemu is no different than perf trying to
> instrument any other userspace widget. Therefore, expecting 'trusted
> enumeration' of instances is just as sensible as using 'trusted ps' and
> 'trusted /var/run/FOO.pid files'.

You are quite mistaken: KVM isnt really a 'random unprivileged application' in
this context, it is clearly an extension of system/kernel services.

( Which can be seen from the simple fact that what started the discussion was
'how do we get /proc/kallsyms from the guest'. I.e. an extension of the
existing host-space /proc/kallsyms was desired. )

In that sense the most natural 'extension' would be the solution i mentioned a
week or two ago: to have a (read only) mount of all guest filesystems, plus a
channel for profiling/tracing data. That would make symbol parsing easier and
it's what extends the existing 'host space' abstraction in the most natural
way.

( It doesnt even have to be done via the kernel - Qemu could implement that
via FUSE for example. )

As a second best option a 'symbol server' might be used too.

Thanks,

Ingo


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