From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 2EABA350291 for ; Tue, 13 Jan 2026 17:03:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768323816; cv=none; b=DARtwcj16O9TTjG0g3UCTeXWMVs+qjMvYcU6lZmxjVreIhazZBDYz8+eZU+5TetNDUuh2/H0KZeWoHLIGfuC3EhXFdzy4Fn63ZzibF2J7DVX7ixELZkWHIRBOp3K/p2kV9ZXI3M5/3swYp6NHa7VdbGjjSy/2vpqiqOscf7gpxw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768323816; c=relaxed/simple; bh=lrdbt2o24sdBLfhOSzTNromel7XSWRN5qQXf5Fx7W30=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=rbOdCPo33X0d/nDJsfmsyILLky82BaMSC3IGCJgw7U4MLDjJ0cM+jkVLCPfk5L1yz64ErCGQJb34RAbaQPPqpzohKI7Dwt5VuJXy4sKEJitDUOu1xcd+xRrr4zX8vx5f2dygwltzsioRANVJRKUcarO2UUXmKTWikPlg9GhokPY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 8D421497; Tue, 13 Jan 2026 09:03:26 -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 26FA03F632; Tue, 13 Jan 2026 09:03:30 -0800 (PST) Date: Tue, 13 Jan 2026 17:03:28 +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> <43ec5e82-88a6-4c7b-983a-b5aa32673913@linaro.org> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <43ec5e82-88a6-4c7b-983a-b5aa32673913@linaro.org> Hi James, On Tue, Jan 13, 2026 at 03:00:34PM +0000, James Clark wrote: > > > On 13/01/2026 10:25 am, Alexandru Elisei wrote: > > 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. > > No, definitely not something that needs to be done at this point. > > > > > > > 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 > > Nice, if it's not too horrible we should try it. If it turns out to be > horrible then yes we'll have to ask people to change their configs. It was only a few lines of code: +void kvm_spe_add_instance(struct arm_spe_pmu *arm_spu) +{ + struct arm_spu_entry *entry; + + guard(mutex)(&arm_spus_lock); + + entry = kmalloc(sizeof(*entry), GFP_KERNEL); + if (!entry) + return; + + entry->arm_spu = arm_spu; + list_add_tail(&entry->link, &arm_spus); +} +EXPORT_SYMBOL_FOR_MODULES(kvm_spe_add_instance, "arm_spe_pmu"); + +void kvm_spe_remove_instance(struct arm_spe_pmu *arm_spu) +{ + struct arm_spu_entry *entry, *tmp; + + guard(mutex)(&arm_spus_lock); + + if (list_empty(&arm_spus)) + return; + + list_for_each_entry_safe(entry, tmp, &arm_spus, link) { + if (entry->arm_spu == arm_spu) { + list_del(&entry->link); + break; + } + } +} +EXPORT_SYMBOL_FOR_MODULES(kvm_spe_remove_instance, "arm_spe_pmu"); + +bool kvm_supports_spe(void) +{ + return !list_empty(&arm_spus); +} Thanks, Alex