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 4F543CD37AC for ; Mon, 11 May 2026 11:52:31 +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:MIME-Version:Content-Type: References:In-Reply-To:Date:Cc:To:From:Subject:Message-ID: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=e2Q/6FUgIqmoVeEngQY8YXWYUbm9HsL7H/cZXEz4tKk=; b=3nvDNBX4mKA1GgOJaZemmqwXvT 2G3HGGVzthfjLqcNZDcJSmnAn7fNDiTjnbnJxU+/LxrEP4Fp0Ri8HoSVH885qyUbec+qgLZynkukU QNN9FmyJELnVTMktfo1p5UbeihaidiDWZxKQIpA51IeCX5EpUMcoXrqVWSej1otW/57fCONgaA/IC QnRj1JLtEyzG9zelNeGAeBQUMFgNN5AneWoQ6wzZqqoq97/eGaZLJgJo9aBCEmAgzwzaCoSj915jg OXdGkkNag0ITmC3uO76dnjuiKBWKubfreS1R23DOTlexvkkinMuy7R7O/7XWjkw1xfJZRLNJF+f6J Gm9xF4zA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMPBR-0000000DP1M-0sqH; Mon, 11 May 2026 11:52:25 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMPBQ-0000000DP0u-1LjV for linux-arm-kernel@bombadil.infradead.org; Mon, 11 May 2026 11:52:24 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=e2Q/6FUgIqmoVeEngQY8YXWYUbm9HsL7H/cZXEz4tKk=; b=BCvLQ0+AfZuXEsIbl1IA/5focF e/oBeNaZpnx/T8g5tfeXIp9SKrFuwrs4An6BP0zBPvNCYtCpzXqcSfETC1qEiyYEhwrdYgiT9WfHE +MKcvIithy1uJFE2KH3vWl2TIpxe/IvMZLfJpl/u87JOFP+annkoIWg3lbdgsDg2reiMsRfsIklhI 4ZLmbgowR3xFYZUE3peG8GhMe2ZPa4OB3Rh4hMuxF3Y8Vn5axy6HK/OlAQh1bdMBEmp74KCDCxPxr 23W0PM40808qWDLkNwetICKrhb9XIr3qVOKaFLy1XXtZ7QOur6FqXMf83q7+jRCeCjok6Xryk7Eou L86kJ4dg==; Received: from 54-240-197-234.amazon.com ([54.240.197.234] helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMPBL-000000087aT-18FB; Mon, 11 May 2026 11:52:19 +0000 Message-ID: Subject: Re: [RFC PATCH] KVM: arm64: vgic: Skip vCPU trylock for pre-init register access From: David Woodhouse To: Yao Yuan Cc: Marc Zyngier , Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Sascha Bischoff , Paolo Bonzini , Jonathan Cameron , Eric Auger , Raghavendra Rao Ananta , Maxim Levitsky , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org Date: Mon, 11 May 2026 12:52:17 +0100 In-Reply-To: <3ijnoudux4iyddpd25yr53s2zqrslplixjqfod225ix43y4z4a@heoklijpq45f> References: <6564c8b967948e30a8d3f35b6ef5de79dd5feeb7.camel@infradead.org> <3ijnoudux4iyddpd25yr53s2zqrslplixjqfod225ix43y4z4a@heoklijpq45f> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-yWimpU8JH7vb3px7gK6r" User-Agent: Evolution 3.52.3-0ubuntu1.1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html 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 --=-yWimpU8JH7vb3px7gK6r Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-05-11 at 19:03 +0800, Yao Yuan wrote: >=20 > > + /* > > + * If the VGIC becomes initialized between the above check and taking > > + * config_lock, drop config_lock to lock the VCPUS. > > + */ > > + if (vgic_initialized(dev->kvm) && !vcpus_locked) { > > + mutex_unlock(&dev->kvm->arch.config_lock); > > + if (kvm_trylock_all_vcpus(dev->kvm)) { > > + mutex_unlock(&dev->kvm->lock); > > + return -EBUSY; > > + } > > + mutex_lock(&dev->kvm->arch.config_lock); > > + vcpus_locked =3D true; >=20 > Question: Why not do vgic_initialized(dev->kvm) and > reg_allowed_pre_init(attr) checking after take config_lock, > and depends on the checking result to decide trylocak all > vcpus or not ? Yeah... I didn't like that either. If we attempt to take the vCPU locks while *holding* config_lock, that gives us a classic ABBA deadlock with the code paths that hold the vCPU locks first. Well, maybe it doesn't because we'd only be doing a *trylock*... but lockdep wouldn't know that and it's fugly as hell. So I settled on the above. The long term fix, as I noted, is to kill kvm_trylock_all_vcpus() with fire in *all* cases, not just the !vgic_initialized() case. On x86 we'd do this with something like KVM_REQ_MCLOCK_INPROGRESS to kick vCPUs out of guest mode to prevent concurrent access. Which isn't my favourite code *either* but at least it works without imposing the awfulness on the userspace API. --=-yWimpU8JH7vb3px7gK6r Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 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