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 678BAD0C83D for ; Tue, 13 Jan 2026 10:26:14 +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=uJHiZVZpIf4HEksYHlw9v05MX5yOg1BSih2wUHlS6WI=; b=O8bPC4+cwU9pRRt6ab26CR2cSA JANm97T7BgUwvUDJzkjLxqoCUsErk6X/nrnqYsmm3UGHomBOQakjOLcYnXLQ4H7ndlCjJKTQJfO1q bO2oyQThfvDIKMQTfokH/dpHk0ypN7z79bi6kgH4tNZVXqpp3TPKisIBHD/QMW+KTUYJ0CVzDpaxA QiRqsLaZBswlhou9R47aa6KlYmUhCQcSiOtPfW8NdWzZ/Ygs6Hjyc92wFBt+LGYgdTRjQ4jsaXyB5 Fm61NOQnIuGKgC7ZN4tXKALKX4Oht0FfE/PPs6rE9oFdR4LZ7yeJ9BaYIpx5XhdeMS9beaiyZ9YDG fTjovXhQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vfbat-00000006u0S-3dfW; Tue, 13 Jan 2026 10:25:47 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vfbar-00000006tzq-3fcp for linux-arm-kernel@lists.infradead.org; Tue, 13 Jan 2026 10:25:46 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 89B15497; Tue, 13 Jan 2026 02:25:36 -0800 (PST) Received: from raptor (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 150023F694; Tue, 13 Jan 2026 02:25:40 -0800 (PST) Date: Tue, 13 Jan 2026 10:25:37 +0000 From: Alexandru Elisei To: James Clark Cc: mark.rutland@arm.com, james.morse@arm.com, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, will@kernel.org, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Subject: Re: [RFC PATCH v6 03/35] KVM: arm64: Add CONFIG_KVM_ARM_SPE Kconfig option Message-ID: References: <20251114160717.163230-1-alexandru.elisei@arm.com> <20251114160717.163230-4-alexandru.elisei@arm.com> <1aaffbcd-ba0c-4371-80d7-ce59ac7f13a9@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260113_022545_989885_565FA4A9 X-CRM114-Status: GOOD ( 44.93 ) 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 Hi James, On Mon, Jan 12, 2026 at 03:18:53PM +0000, Alexandru Elisei wrote: > Hi James, > > On Mon, Jan 12, 2026 at 12:09:19PM +0000, James Clark wrote: > > > > > > On 12/01/2026 11:26 am, Alexandru Elisei wrote: > > > Hi James, > > > > > > On Fri, Jan 09, 2026 at 04:29:50PM +0000, James Clark wrote: > > > > > > > > > > > > On 14/11/2025 4:06 pm, Alexandru Elisei wrote: > > > > > Add a new configuration option that will be used for KVM SPE emulation. > > > > > CONFIG_KVM_ARM_SPE depends on the SPE driver being builtin because: > > > > > > > > > > 1. The SPE driver maintains a cpumask of physical CPUs that support SPE, > > > > > and that will be used by KVM to emulate SPE on heterogeneous systems. > > > > > > > > > > 2. KVM will rely on the SPE driver enabling the SPE interrupt at the GIC > > > > > level. > > > > > > > > > > Signed-off-by: Alexandru Elisei > > > > > --- > > > > > arch/arm64/kvm/Kconfig | 8 ++++++++ > > > > > 1 file changed, 8 insertions(+) > > > > > > > > > > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig > > > > > index 4f803fd1c99a..31388b5b2655 100644 > > > > > --- a/arch/arm64/kvm/Kconfig > > > > > +++ b/arch/arm64/kvm/Kconfig > > > > > @@ -83,4 +83,12 @@ config PTDUMP_STAGE2_DEBUGFS > > > > > If in doubt, say N. > > > > > +config KVM_ARM_SPE > > > > > + bool > > > > > + depends on KVM && ARM_SPE_PMU=y > > > > > > > > I think the most common configuration is module, so requiring built-in isn't > > > > great. If there's any way of avoiding it, even if it costs a little bit of > > > > pain, it would be good for adoption. > > > > > > I'm not sure how that could be done. You need the buffer maintenance interrupt > > > to do a VM exit so KVM can inject the virtual interrupt back into the guest, > > > otherwise there's a possibility of a large blackout window when the buffer is > > > full. > > > > Without expanding more I don't see how injecting an interrupt is strongly > > related to the way it's compiled. > > It's the SPE driver that enables the hardware interrupt in the GIC. > > > > > > > > > The code also relies on the SPE driver to probe all the SPUs in the system. > > > > > > > I don't think that's a hard requirement either, the KVM code could fairly > > easily do this itself. Inline the common code and put it in a shared header > > etc. I'm sure there are lots of creative ways to flip the dependencies that > > might not be "proper" software engineering. But like I said a bit of pain > > here might be beneficial overall. > > Userspace gets the maximum buffer size and the SPE instance id from the sysfs > files. > > So KVM would have to parse the DTB, read PMBIDR_EL1 and PMSIDR_EL1 on each > physical CPU in the system, create the sysfs files (at least 'type', 'cpumask' > and 'max_buffer_size' or whatever that ends up being named) and install a dummy > IRQ handler before the SPE driver is loaded. > > Then when the SPE driver is loaded, the driver would have to remove the IRQ > handler and install its own, and create the rest of the sysfs files. > > That's assuming that the SPE driver is loaded after KVM. I think it's possible > for the SPE driver to be loaded first. So both KVM and SPE would have be able to > communicate with one other in case the other probed first. > > Is having to change the symbol ARM_SPE_PMU from 'm' to 'y' worth this? I am not > convinced and I don't see much use for going down this rabbit hole at this stage > of the series. > > Also, since KVM would partially populate the sysfs directory for each SPE > instance, would it be reasonable for software to assume that the SPE driver is > loaded based on this and start using it? For example, how does perf detect the > presence of the arm_spe event? Coming back to this, I do understand and agree with your concerns, so after talking in private I came up with exporting the symbol kvm_host_spe_init() - that's the function which adds the SPE instance to the list of SPUs. With this change the SPE driver can be built as a module, but SPE for a VM is available only as long as the driver is loaded. I hope that improves the adoption story :) Thanks, Alex