From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 15792C28B30 for ; Thu, 20 Mar 2025 17:53:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=T+nATEGPwxa2fwpxH+oyQjnjtO3F1E78wE3wcZDa8qM=; b=a29sCwCqEwa+3ve9t5nXRPwPBb WQMm7jnfivAzoPNArZLpT1vZWoS+YLbE8kFPA3yqIJquax7e/wNXYssMzbI8d/rtJQNivwxpv9qSM vgPclTR6Y1+N/iFZbxTLlG1l5xrPpqFcMoM0twjVdWcWs9FJAzHCFMJL/uC6fus7bG6U6BGmtXLyJ 93YI1gRyG7nclHMO0B/GrhaW46jcTapj2I1BbzM0QASFdYE9qyB8GY4xQNHVwuuxFxw2UuyWOcBRz g9OvOqUf7t2kHjK4wqTgymlPC3xpmD4aYQD3CtkFrxbYZzARm2hYclmVVC+YKRhfHwPCARhE6e80j N8qxBhkg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1tvK5N-0000000Cs6M-3KLd; Thu, 20 Mar 2025 17:53:41 +0000 Received: from out-171.mta1.migadu.com ([95.215.58.171]) by bombadil.infradead.org with esmtps (Exim 4.98 #2 (Red Hat Linux)) id 1tvJwN-0000000CqT2-1lYP for linux-arm-kernel@lists.infradead.org; Thu, 20 Mar 2025 17:44:24 +0000 Date: Thu, 20 Mar 2025 17:44:03 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1742492660; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=T+nATEGPwxa2fwpxH+oyQjnjtO3F1E78wE3wcZDa8qM=; b=JTCyBZ6mjiYhZGkv782PMWnYrzrKTbJ3koWgqojfxmL4ewWviXPY0IyXE8HjwsaLSf10Wc NlnV38ZtLi9zY89NSxiMVnXzGuD9xU4Er7f+X8odYLu78qoIK7anhWcMI6CtIv0I/jpVBR ltETHPFcSerr+Yyx7eDLcRRzlzAunQw= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Marc Zyngier Cc: Akihiko Odaki , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Kees Cook , "Gustavo A. R. Silva" , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, devel@daynix.com Subject: Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs Message-ID: References: <86plidmjwh.wl-maz@kernel.org> <86o6xxmg87.wl-maz@kernel.org> <86msdhmemw.wl-maz@kernel.org> <86ldt0n9w1.wl-maz@kernel.org> <86h63om54p.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <86h63om54p.wl-maz@kernel.org> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250320_104423_603678_C8B06F0B X-CRM114-Status: GOOD ( 26.01 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Mar 20, 2025 at 09:19:02AM +0000, Marc Zyngier wrote: > On Wed, 19 Mar 2025 18:51:28 +0000, Oliver Upton wrote: > > I'm at least willing to plug my nose and do the following: > > > > 1) When the VMM does not specify a vPMU type: > > > > - We continue to present the 'default' PMU (including event counters) > > to the VM > > > > - KVM ensures that the fixed CPU cycle counter works on any PMUv3 > > implementation in the system, even if it is different from the > > default > > > > - Otherwise, event counters will only count on the default > > implementation and will not count on different PMUs > > I think this is confusing. The CC is counting, but nothing else, and > people using the cycle counters in conjunction with other events (a > very common use case) will not be able to correlate things correctly. > The current behaviour is, for all its sins, at least consistent. You of course have a good point. What Windows is doing is definitely an outlier. > > > > 2) Implement your suggestion of a UAPI where the VMM can select a PMU > > that only has the CPU cycle counter and works on any PMUv3 > > implementation. > > > > Either way KVM will need to have some special case handling of the fixed > > CPU cycle counter. That'd allow users to actually run Windows *now* and > > provide a clear mechanism for userspace to present a less-broken vPMU if > > it cares. > > Honestly, I don't care about one guest or another. My point is that if > we are changing the behaviour of the PMU to deal with this sort of > things, then it has to be a userspace buy-in. I'm fine with just the user buy-in then. But I still do care about the guest compatibility issue, especially since the end user of all this crap is unlikely to know/care about the fine details of the implementation. So, Akihiko, I would *greatly* appreciate it if you propose a complete solution to the problem, including the KVM and VMM patches to make it all work. Thanks, Oliver