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From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	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, Paolo Bonzini <pbonzini@redhat.com>,
	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 05/15] perf: Track guest callbacks on a per-CPU basis
Date: Fri, 27 Aug 2021 09:15:17 +0200	[thread overview]
Message-ID: <YSiRBQQE7md7ZrNC@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210827005718.585190-6-seanjc@google.com>

On Thu, Aug 26, 2021 at 05:57:08PM -0700, Sean Christopherson wrote:
> Use a per-CPU pointer to track perf's guest callbacks so that KVM can set
> the callbacks more precisely and avoid a lurking NULL pointer dereference.

I'm completely failing to see how per-cpu helps anything here...

> On x86, KVM supports being built as a module and thus can be unloaded.
> And because the shared callbacks are referenced from IRQ/NMI context,
> unloading KVM can run concurrently with perf, and thus all of perf's
> checks for a NULL perf_guest_cbs are flawed as perf_guest_cbs could be
> nullified between the check and dereference.

No longer allowing KVM to be a module would be *AWESOME*. I detest how
much we have to export for KVM :/

Still, what stops KVM from doing a coherent unreg? Even the
static_call() proposed in the other patch, unreg can do
static_call_update() + synchronize_rcu() to ensure everybody sees the
updated pointer (would require a quick audit to see all users are with
preempt disabled, but I think your using per-cpu here already imposes
the same).



  reply	other threads:[~2021-08-27  7:19 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27  0:57 [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks Sean Christopherson
2021-08-27  0:57 ` [PATCH 01/15] KVM: x86: Register perf callbacks after calling vendor's hardware_setup() Sean Christopherson
2021-08-27  0:57 ` [PATCH 02/15] KVM: x86: Register Processor Trace interrupt hook iff PT enabled in guest Sean Christopherson
2021-08-27  0:57 ` [PATCH 03/15] perf: Stop pretending that perf can handle multiple guest callbacks Sean Christopherson
2021-08-27  0:57 ` [PATCH 04/15] perf: Force architectures to opt-in to " Sean Christopherson
2021-08-27  0:57 ` [PATCH 05/15] perf: Track guest callbacks on a per-CPU basis Sean Christopherson
2021-08-27  7:15   ` Peter Zijlstra [this message]
2021-08-27 14:49     ` Sean Christopherson
2021-08-27 14:56       ` Peter Zijlstra
2021-08-27 15:22         ` Sean Christopherson
2021-08-27  0:57 ` [PATCH 06/15] KVM: x86: Register perf callbacks only when actively handling interrupt Sean Christopherson
2021-08-27  7:21   ` Peter Zijlstra
2021-08-27  0:57 ` [PATCH 07/15] KVM: Use dedicated flag to track if KVM is handling an NMI from guest Sean Christopherson
2021-08-27  7:30   ` Peter Zijlstra
2021-08-27 14:58     ` Sean Christopherson
2021-08-27  0:57 ` [PATCH 08/15] KVM: x86: Drop current_vcpu in favor of kvm_running_vcpu Sean Christopherson
2021-08-27  0:57 ` [PATCH 09/15] KVM: arm64: Register/unregister perf callbacks at vcpu load/put Sean Christopherson
2021-08-27  0:57 ` [PATCH 10/15] KVM: Move x86's perf guest info callbacks to generic KVM Sean Christopherson
2021-08-27  0:57 ` [PATCH 11/15] KVM: x86: Move Intel Processor Trace interrupt handler to vmx.c Sean Christopherson
2021-08-27  7:34   ` Peter Zijlstra
2021-08-27  0:57 ` [PATCH 12/15] KVM: arm64: Convert to the generic perf callbacks Sean Christopherson
2021-08-27  0:57 ` [PATCH 13/15] KVM: arm64: Drop perf.c and fold its tiny bit of code into pmu.c Sean Christopherson
2021-08-27  0:57 ` [PATCH 14/15] perf: Disallow bulk unregistering of guest callbacks and do cleanup Sean Christopherson
2021-08-27  0:57 ` [PATCH 15/15] perf: KVM: Indicate "in guest" via NULL ->is_in_guest callback Sean Christopherson
2021-08-27  6:52 ` [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks Like Xu
2021-08-27  7:44   ` Peter Zijlstra
2021-08-27  8:01     ` Like Xu
2021-08-27 10:47       ` Peter Zijlstra

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