From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.8]) (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 316011CD06; Tue, 16 Apr 2024 05:12:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.8 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713244329; cv=none; b=SAvSMfoC7rS0qmlxu1pYZG2SesW87G34D4EoUSvkMBHUiD7rBP6KjECikBVBPh+0Id2QeVTnAaLrVZu0sy726zJoyPqmIfgE3tjQ0LQQhKe+7rZaXYbyoNqSySrFP+uSGo00kX6zBPBwNJFgZidlzARBOIHnNcmJpctKvpKUEBQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713244329; c=relaxed/simple; bh=Xk702F0LwWoR8IwEhI5DFA1b/JYz97xwQQDY6dFv2D0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Q9cmMabzss4T8wGkfRdXZS4YUWwR8wTL63bAjo43VI5bkQY5TUv1gJayuEUpGjoowLCfQuDjhZGBn0IknEU08kHKSvkB5V6OgFiussQmbsM0T3wuP1cl/260cVhWSfEBzs4y+mOCttYmPgXKPyQtDAe8Q+KBp3kvbirelAnDxok= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com; spf=none smtp.mailfrom=linux.intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=SPDuRXQf; arc=none smtp.client-ip=192.198.163.8 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="SPDuRXQf" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1713244327; x=1744780327; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=Xk702F0LwWoR8IwEhI5DFA1b/JYz97xwQQDY6dFv2D0=; b=SPDuRXQfPnneAtiIunkL2WyCSAYe4nWwTWZelvNb8iTEfQRTcoFMLYKl cgiiDqKDzTETF0eiD1NXPVnvYdC45CPdqL/U3s2pYQJj6iF1RvEkCWNxz AKmRque/0YzPuHTvytkE1TMp+Cv1LEX5irC3MbBqmy+EEM1AKJmRY6uXP oZ8Yf9rCVjzLgb9NAvrMkI3g0zK+NT+3mJ9CjoiCMZbQPZFE7sw0IpzW0 BYx5+81rDyW+Zy8FKq3kMHVdbQD47A2HXjoHYtYCw0IzapGonRFEXe7Wz fiy2AnAaKvNdC5gC1SRdsKhYuILyBlW4vuvthxdEWrTRGCvUBwMYuoWL3 A==; X-CSE-ConnectionGUID: em44YNQOTuSzGSubBV8JqQ== X-CSE-MsgGUID: rnLFtWddQVmDXCxNudp8qA== X-IronPort-AV: E=McAfee;i="6600,9927,11045"; a="26176781" X-IronPort-AV: E=Sophos;i="6.07,205,1708416000"; d="scan'208";a="26176781" Received: from orviesa003.jf.intel.com ([10.64.159.143]) by fmvoesa102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Apr 2024 22:12:06 -0700 X-CSE-ConnectionGUID: fwpcdSv+Qg6Ln0RoMBkODw== X-CSE-MsgGUID: onzIOJOaQOSwKu1xjB+Qcg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.07,205,1708416000"; d="scan'208";a="26801136" Received: from unknown (HELO [10.238.128.139]) ([10.238.128.139]) by ORVIESA003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Apr 2024 22:12:02 -0700 Message-ID: <47157187-47d8-424e-9bd1-b98690dfe68a@linux.intel.com> Date: Tue, 16 Apr 2024 13:11:59 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 00/41] KVM: x86/pmu: Introduce passthrough vPM To: Sean Christopherson Cc: pbonzini@redhat.com, peterz@infradead.org, mizhang@google.com, kan.liang@intel.com, zhenyuw@linux.intel.com, dapeng1.mi@linux.intel.com, jmattson@google.com, kvm@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, zhiyuan.lv@intel.com, eranian@google.com, irogers@google.com, samantha.alt@intel.com, like.xu.linux@gmail.com, chao.gao@intel.com References: <20240126085444.324918-1-xiong.y.zhang@linux.intel.com> <9469faf7-1659-4436-848f-53ec01d967f2@linux.intel.com> Content-Language: en-US From: "Zhang, Xiong Y" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/15/2024 11:05 PM, Sean Christopherson wrote: > On Mon, Apr 15, 2024, Xiong Y Zhang wrote: >> On 4/13/2024 2:32 AM, Sean Christopherson wrote: >>> On Fri, Apr 12, 2024, Xiong Y Zhang wrote: >>>>>> 2. NMI watchdog >>>>>> the perf event for NMI watchdog is a system wide cpu pinned event, it >>>>>> will be stopped also during vm running, but it doesn't have >>>>>> attr.exclude_guest=1, we add it in this RFC. But this still means NMI >>>>>> watchdog loses function during VM running. >>>>>> >>>>>> Two candidates exist for replacing perf event of NMI watchdog: >>>>>> a. Buddy hardlock detector[3] may be not reliable to replace perf event. >>>>>> b. HPET-based hardlock detector [4] isn't in the upstream kernel. >>>>> >>>>> I think the simplest solution is to allow mediated PMU usage if and only if >>>>> the NMI watchdog is disabled. Then whether or not the host replaces the NMI >>>>> watchdog with something else becomes an orthogonal discussion, i.e. not KVM's >>>>> problem to solve. >>>> Make sense. KVM should not affect host high priority work. >>>> NMI watchdog is a client of perf and is a system wide perf event, perf can't >>>> distinguish a system wide perf event is NMI watchdog or others, so how about >>>> we extend this suggestion to all the system wide perf events ? mediated PMU >>>> is only allowed when all system wide perf events are disabled or non-exist at >>>> vm creation. >>> >>> What other kernel-driven system wide perf events are there? >> does "kernel-driven" mean perf events created through >> perf_event_create_kernel_counter() like nmi_watchdog and kvm perf events ? > > By kernel-driven I meant events that aren't tied to a single userspace process > or action. > > E.g. KVM creates events, but those events are effectively user-driven because > they will go away if the associated VM terminates. > >> User can create system wide perf event through "perf record -e {} -a" also, I >> call it as user-driven system wide perf events. Perf subsystem doesn't >> distinguish "kernel-driven" and "user-driven" system wide perf events. > > Right, but us humans can build a list, even if it's only for documentation, e.g. > to provide help for someone to run KVM guests with mediated PMUs, but can't > because there are active !exclude_guest events. > >>>> but NMI watchdog is usually enabled, this will limit mediated PMU usage. >>> >>> I don't think it is at all unreasonable to require users that want optimal PMU >>> virtualization to adjust their environment. And we can and should document the >>> tradeoffs and alternatives, e.g. so that users that want better PMU results don't >>> need to re-discover all the "gotchas" on their own. >>> >>> This would even be one of the rare times where I would be ok with a dmesg log. >>> E.g. if KVM is loaded with enable_mediated_pmu=true, but there are system wide >>> perf events, pr_warn() to explain the conflict and direct the user at documentation >>> explaining how to make their system compatible with mediate PMU usage.> >>>>>> 3. Dedicated kvm_pmi_vector >>>>>> In emulated vPMU, host PMI handler notify KVM to inject a virtual >>>>>> PMI into guest when physical PMI belongs to guest counter. If the >>>>>> same mechanism is used in passthrough vPMU and PMI skid exists >>>>>> which cause physical PMI belonging to guest happens after VM-exit, >>>>>> then the host PMI handler couldn't identify this PMI belongs to >>>>>> host or guest. >>>>>> So this RFC uses a dedicated kvm_pmi_vector, PMI belonging to guest >>>>>> has this vector only. The PMI belonging to host still has an NMI >>>>>> vector. >>>>>> >>>>>> Without considering PMI skid especially for AMD, the host NMI vector >>>>>> could be used for guest PMI also, this method is simpler and doesn't >>>>> >>>>> I don't see how multiplexing NMIs between guest and host is simpler. At best, >>>>> the complexity is a wash, just in different locations, and I highly doubt it's >>>>> a wash. AFAIK, there is no way to precisely know that an NMI came in via the >>>>> LVTPC. >>>> when kvm_intel.pt_mode=PT_MODE_HOST_GUEST, guest PT's PMI is a multiplexing >>>> NMI between guest and host, we could extend guest PT's PMI framework to >>>> mediated PMU. so I think this is simpler. >>> >>> Heh, what do you mean by "this"? Using a dedicated IRQ vector, or extending the >>> PT framework of multiplexing NMI? >> here "this" means "extending the PT framework of multiplexing NMI". > > The PT framework's multiplexing is just as crude as regular PMIs though. Perf > basically just asks KVM: is this yours? And KVM simply checks that the callback > occurred while KVM_HANDLING_NMI is set. > > E.g. prior to commit 11df586d774f ("KVM: VMX: Handle NMI VM-Exits in noinstr region"), > nothing would prevent perf from miscontruing a host PMI as a guest PMI, because > KVM re-enabled host PT prior to servicing guest NMIs, i.e. host PT would be active > while KVM_HANDLING_NMI is set. > > And conversely, if a guest PMI skids past VM-Exit, as things currently stand, the > NMI will always be treated as host PMI, because KVM will not be in KVM_HANDLING_NMI. > KVM's emulated PMI can (and should) eliminate false positives for host PMIs by > precisely checking exclude_guest, but that doesn't help with false negatives for > guest PMIs, nor does it help with NMIs that aren't perf related, i.e. didn't come > from the LVTPC> > Is a naive implementation simpler? Maybe. But IMO, multiplexing NMI and getting > all the edge cases right is more complex than using a dedicated vector for guest > PMIs, as the latter provides a "hard" boundary and allows the kernel to _know_ that > an interrupt is for a guest PMI. >Totally agree the complex to fix multiplexing NMI corner case. Thanks for explanation.