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

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>, Guo Ren <guoren@kernel.org>,
	Nick Hu <nickhu@andestech.com>, Greentime Hu <green.hu@gmail.com>,
	Vincent Chen <deanbo422@gmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, linux-csky@vger.kernel.org,
	linux-riscv@lists.infradead.org, kvm@vger.kernel.org,
	xen-devel@lists.xenproject.org,
	Artem Kashkanov <artem.kashkanov@intel.com>,
	Like Xu <like.xu.linux@gmail.com>,
	Zhu Lingshan <lingshan.zhu@intel.com>
Subject: Re: [PATCH v2 05/13] perf: Force architectures to opt-in to guest callbacks
Date: Tue, 21 Sep 2021 21:29:48 +0000	[thread overview]
Message-ID: <YUpOzFW3K3iJAoWa@google.com> (raw)
In-Reply-To: <8ee13a69-f2c4-2413-2d6c-b6c0a559286e@redhat.com>

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.