From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A86611B29B; Wed, 11 Oct 2023 16:45:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dtxmhkay" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2CC1EC433C7; Wed, 11 Oct 2023 16:45:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1697042738; bh=Gszkr6ytwYAWj77761HPc4xxX6XKWbdeQEVhpa208hM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=dtxmhkayGxf0TZptHCQp2gsnsvH2XldyeRAADcrby7Ysmpyl05/MP5u9QgwOA2o1P DbRv0VeNO+P7UFFRxSMDOTTK4j213srQCPgK4OQjOuBM17Lz/Yq+6djsegBECYCi9z 8i9dJ1K5qKXeOYlMwUeAohGPHNeW3ZmwEM3WWMFz5jz81V7z77a7umSHScM083mgmS SG1xL3eEjuLWPsHm8PxGJn60d+dAWe03wsyNWeSs+K4Hz1L1ZSH1Hs+l9HqLJnGFak +Jy2abvU6Ig10vflOk7hganeEUNBbh1oKJFB0fltA2MmUkYjO9/RSoY1M5oDw5Z6Q0 P2KEF0Itn0vKQ== Received: from [104.132.1.97] (helo=wait-a-minute.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1qqcL1-003DWa-8a; Wed, 11 Oct 2023 17:45:35 +0100 Date: Wed, 11 Oct 2023 17:45:29 +0100 Message-ID: <8734yhm7km.wl-maz@kernel.org> From: Marc Zyngier To: Tianyi Liu Cc: seanjc@google.com, pbonzini@redhat.com, peterz@infradead.org, mingo@redhat.com, acme@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, namhyung@kernel.org, irogers@google.com, adrian.hunter@intel.com Subject: Re: [PATCH v2 0/5] perf: KVM: Enable callchains for guests In-Reply-To: References: User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/28.2 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 104.132.1.97 X-SA-Exim-Rcpt-To: i.pear@outlook.com, seanjc@google.com, pbonzini@redhat.com, peterz@infradead.org, mingo@redhat.com, acme@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, namhyung@kernel.org, irogers@google.com, adrian.hunter@intel.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Sun, 08 Oct 2023 15:48:17 +0100, Tianyi Liu wrote: > > Hi there, > > This series of patches enables callchains for guests (used by perf kvm), > which holds the top spot on the perf wiki TODO list [1]. This allows users > to perform guest OS callchain or performance analysis from external > using PMU events. > > The event processing flow is as follows (shown as backtrace): > #0 kvm_arch_vcpu_get_frame_pointer / kvm_arch_vcpu_read_virt (per arch) > #1 kvm_guest_get_frame_pointer / kvm_guest_read_virt > > #2 perf_guest_get_frame_pointer / perf_guest_read_virt > #3 perf_callchain_guest > #4 get_perf_callchain > #5 perf_callchain > > Between #0 and #1 is the interface between KVM and the arch-specific > impl, while between #1 and #2 is the interface between Perf and KVM. > The 1st patch implements #0. The 2nd patch extends interfaces between #1 > and #2, while the 3rd patch implements #1. The 4th patch implements #3 > and modifies #4 #5. The last patch is for userspace utils. > > Since arm64 hasn't provided some foundational infrastructure (interface > for reading from a virtual address of guest), the arm64 implementation > is stubbed for now because it's a bit complex, and will be implemented > later. I hope you realise that such an "interface" would be, by definition, fragile and very likely to break in a subtle way. The only existing case where we walk the guest's page tables is for NV, and even that is extremely fragile. Given that, I really wonder why this needs to happen in the kernel. Userspace has all the required information to interrupt a vcpu and walk its current context, without any additional kernel support. What are the bits here that cannot be implemented anywhere else? > > Tested with both 32-bit and 64-bit guest operating systems / unikernels, > that `perf script` could correctly show the certain callchains. > FlameGraphs can also be generated with this series of patches and [2]. > > Any feedback will be greatly appreciated. > > [1] https://perf.wiki.kernel.org/index.php/Todo > [2] https://github.com/brendangregg/FlameGraph > > v1: > https://lore.kernel.org/kvm/SYYP282MB108686A73C0F896D90D246569DE5A@SYYP282MB1086.AUSP282.PROD.OUTLOOK.COM/ > > Changes since v1: > - v1 only includes partial KVM modifications, while v2 is a complete > implementation. Also updated based on Sean's feedback. > > Tianyi Liu (5): > KVM: Add arch specific interfaces for sampling guest callchains > perf kvm: Introduce guest interfaces for sampling callchains > KVM: implement new perf interfaces > perf kvm: Support sampling guest callchains > perf tools: Support PERF_CONTEXT_GUEST_* flags > > arch/arm64/kvm/arm.c | 17 +++++++++ Given that there is more to KVM than just arm64 and x86, I suggest that you move the lack of support for this feature into the main KVM code. Thanks, M. -- Without deviation from the norm, progress is not possible.