linux-kernel - Re: [PATCH v2 05/13] perf: Force architectures to opt-in to guest callbacks

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Message-ID: <YUpOzFW3K3iJAoWa@google.com>
Date:   Tue, 21 Sep 2021 21:29:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH v2 05/13] perf: Force architectures to opt-in to guest
 callbacks

On Tue, Sep 21, 2021, Paolo Bonzini wrote:
> On 28/08/21 21:47, Peter Zijlstra wrote:
> > > +config HAVE_GUEST_PERF_EVENTS
> > > +	bool
> > 	depends on HAVE_KVM
> 
> It won't really do anything, since Kconfig does not detects conflicts
> between select' and 'depends on' clauses.

It does throw a WARN, though the build doesn't fail.

WARNING: unmet direct dependencies detected for HAVE_GUEST_PERF_EVENTS
  Depends on [n]: HAVE_KVM [=n] && HAVE_PERF_EVENTS [=y]
  Selected by [y]:
  - ARM64 [=y]

WARNING: unmet direct dependencies detected for HAVE_GUEST_PERF_EVENTS
  Depends on [n]: HAVE_KVM [=n] && HAVE_PERF_EVENTS [=y]
  Selected by [y]:
  - ARM64 [=y]

WARNING: unmet direct dependencies detected for HAVE_GUEST_PERF_EVENTS
  Depends on [n]: HAVE_KVM [=n] && HAVE_PERF_EVENTS [=y]
  Selected by [y]:
  - ARM64 [=y]

> Rather, should the symbol be selected by KVM, instead of ARM64 and X86?

By KVM, you mean KVM in arm64 and x86, correct?  Because HAVE_GUEST_PERF_EVENTS
should not be selected for s390, PPC, or MIPS.

Oh, and Xen also uses the callbacks on x86, which means the HAVE_KVM part is
arguabably wrong, even though it's guaranteed to be true for the XEN_PV case.
I'll drop that dependency and send out a separate series to clean up the arm64
side of HAVE_KVM.

The reason I didn't bury HAVE_GUEST_PERF_EVENTS under KVM (and XEN_PV) is that
there are number of references to the callbacks throught perf and I didn't want
to create #ifdef hell.

But I think I figured out a not-awful solution.  If there are wrappers+stubs for
the guest callback users, then the new Kconfig can be selected on-demand instead
of unconditionally by arm64 and x86.  That has the added bonus of eliminating
the relevant code paths for !KVM (and !XEN_PV on x86), with or without static_call.
It also obviates the needs for __KVM_WANT_GUEST_PERF_EVENTS or whatever I called
that thing.

It more or less requires defining the static calls in generic perf, but I think
that actually ends up being good thing as it consolidates more code without
introducing more #ifdefs.  The diffstats for the static_call() conversions are
also quite nice.

 include/linux/perf_event.h | 28 ++++++----------------------
 kernel/events/core.c       | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 22 deletions(-)

I'll try to get a new version out today or tomorrow.