Re: [PATCH 10/10] perf/doc: update design.txt for exclude_{host|guest} flags

[Andrew Murray <andrew.murray@arm.com>]

On Wed, Dec 12, 2018 at 09:07:42AM +0100, Christoffer Dall wrote:
> On Tue, Dec 11, 2018 at 01:59:03PM +0000, Andrew Murray wrote:
> > On Tue, Dec 11, 2018 at 10:06:53PM +1100, Michael Ellerman wrote:
> > > [ Reviving old thread. ]
> > > 
> > > Andrew Murray <andrew.murray@arm.com> writes:
> > > > On Tue, Nov 20, 2018 at 10:31:36PM +1100, Michael Ellerman wrote:
> > > >> Andrew Murray <andrew.murray@arm.com> writes:
> > > >> 
> > > >> > Update design.txt to reflect the presence of the exclude_host
> > > >> > and exclude_guest perf flags.
> > > >> >
> > > >> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > >> > ---
> > > >> >  tools/perf/design.txt | 4 ++++
> > > >> >  1 file changed, 4 insertions(+)
> > > >> >
> > > >> > diff --git a/tools/perf/design.txt b/tools/perf/design.txt
> > > >> > index a28dca2..7de7d83 100644
> > > >> > --- a/tools/perf/design.txt
> > > >> > +++ b/tools/perf/design.txt
> > > >> > @@ -222,6 +222,10 @@ The 'exclude_user', 'exclude_kernel' and 'exclude_hv' bits provide a
> > > >> >  way to request that counting of events be restricted to times when the
> > > >> >  CPU is in user, kernel and/or hypervisor mode.
> > > >> >  
> > > >> > +Furthermore the 'exclude_host' and 'exclude_guest' bits provide a way
> > > >> > +to request counting of events restricted to guest and host contexts when
> > > >> > +using virtualisation.
> > > >> 
> > > >> How does exclude_host differ from exclude_hv ?
> > > >
> > > > I believe exclude_host / exclude_guest are intented to distinguish
> > > > between host and guest in the hosted hypervisor context (KVM).
> > > 
> > > OK yeah, from the perf-list man page:
> > > 
> > >            u - user-space counting
> > >            k - kernel counting
> > >            h - hypervisor counting
> > >            I - non idle counting
> > >            G - guest counting (in KVM guests)
> > >            H - host counting (not in KVM guests)
> > > 
> > > > Whereas exclude_hv allows to distinguish between guest and
> > > > hypervisor in the bare-metal type hypervisors.
> > > 
> > > Except that's exactly not how we use them on powerpc :)
> > > 
> > > We use exclude_hv to exclude "the hypervisor", regardless of whether
> > > it's KVM or PowerVM (which is a bare-metal hypervisor).
> > > 
> > > We don't use exclude_host / exclude_guest at all, which I guess is a
> > > bug, except I didn't know they existed until this thread.
> > > 
> > > eg, in a KVM guest:
> > > 
> > >   $ perf record -e cycles:G /bin/bash -c "for i in {0..100000}; do :;done"
> > >   $ perf report -D | grep -Fc "dso: [hypervisor]"
> > >   16
> > > 
> > > 
> > > > In the case of arm64 - if VHE extensions are present then the host
> > > > kernel will run at a higher privilege to the guest kernel, in which
> > > > case there is no distinction between hypervisor and host so we ignore
> > > > exclude_hv. But where VHE extensions are not present then the host
> > > > kernel runs at the same privilege level as the guest and we use a
> > > > higher privilege level to switch between them - in this case we can
> > > > use exclude_hv to discount that hypervisor role of switching between
> > > > guests.
> > > 
> > > I couldn't find any arm64 perf code using exclude_host/guest at all?
> > 
> > Correct - but this is in flight as I am currently adding support for this
> > see [1].
> > 
> > > 
> > > And I don't see any x86 code using exclude_hv.
> > 
> > I can't find any either.
> > 
> > > 
> > > But maybe that's OK, I just worry this is confusing for users.
> > 
> > There is some extra context regarding this where exclude_guest/exclude_host
> > was added, see [2] and where exclude_hv was added, see [3]
> > 
> > Generally it seems that exclude_guest/exclude_host relies upon switching
> > counters off/on on guest/host switch code (which works well in the nested
> > virt case). Whereas exclude_hv tends to rely solely on hardware capability
> > based on privilege level (which works well in the bare metal case where
> > the guest doesn't run at same privilege as the host).
> > 
> > I think from the user perspective exclude_hv allows you to see your overhead
> > if you are a guest (i.e. work done by bare metal hypervisor associated with
> > you as the guest). Whereas exclude_guest/exclude_host doesn't allow you to
> > see events above you (i.e. the kernel hypervisor) if you are the guest...
> > 
> > At least that's how I read this, I've copied in others that may provide
> > more authoritative feedback.
> > 
> > [1] https://lists.cs.columbia.edu/pipermail/kvmarm/2018-December/033698.html
> > [2] https://www.spinics.net/lists/kvm/msg53996.html
> > [3] https://lore.kernel.org/patchwork/patch/143918/
> > 
> 
> I'll try to answer this in a different way, based on previous
> discussions with Joerg et al. who introduced these flags.  Assume no
> support for nested virtualization as a first approximation:
> 
>   If you are running as a guest:
>     - exclude_hv: stop counting events when the hypervisor runs
>     - exclude_host: has no effect
>     - exclude_guest: has no effect
>   
>   If you are running as a host/hypervisor:
>    - exclude_hv: has no effect
>    - exclude_host: only count events when the guest is running
>    - exclude_guest: only count events when the host is running
> 
> With nested virtualization, you get the natural union of the above.
> 
> **This has nothing to do with the design of the hypervisor such as the
> ARM non-VHE KVM which splits its execution across EL1 and EL2 -- those
> are both considered host from the point of view of Linux as a hypervisor
> using KVM, and both considered hypervisor from the point of view of a
> guest.**

For clarity, this is what arm64 currently does (assuming no nesting and
without the current version of this patchset):

   If you are running as a guest (VHE or !VHE host):
     - exclude_hv: has no effect for a KVM guest (filters hypervisor on !VHE
		   bare metal hypervisor guest)
     - exclude_host: has no effect
     - exclude_guest: has no effect
   
   If you are running as a host/hypervisor:
    - exclude_hv: has no effect for VHE (filters EL2 on !VHE)
    - exclude_host: only count events when the guest is running
    - exclude_guest: only count events when the host is running
 
Is this as expected?

Thanks,

Andrew Murray

> 
> 
> Thanks,
> 
>     Christoffer