Re: [PATCH v2 05/13] perf: Force architectures to opt-in to guest callbacks
From: Sean Christopherson <seanjc@google.com>
Date: 2021-09-21 21:30:03
Also in:
kvm, kvmarm, linux-arm-kernel, linux-riscv, lkml, xen-devel
On Tue, Sep 21, 2021, Paolo Bonzini wrote:
On 28/08/21 21:47, Peter Zijlstra wrote:quoted
quoted
+config HAVE_GUEST_PERF_EVENTS + booldepends on HAVE_KVMIt 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.