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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3203C197A0 for ; Thu, 16 Nov 2023 14:21:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231210AbjKPOVp (ORCPT ); Thu, 16 Nov 2023 09:21:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39000 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231205AbjKPOVn (ORCPT ); Thu, 16 Nov 2023 09:21:43 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3F603B7 for ; Thu, 16 Nov 2023 06:21:40 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C8EE7C433C8; Thu, 16 Nov 2023 14:21:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1700144499; bh=raEdXIbHP8Aquq+VfgfjVsJSTdoLci7y4OvBBKOdmaA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=mr2kQJ2MLWeTQ1uTcmCXC1yB8aQe05cgGP6fEoqxxnjcSvIhfBF1sJmJOC52QFD86 05UFHrQfSYU2JUOa/SAqxenYNbUxewYp2pghKmIVMOele0OylQLxQiQZu+lGqGSwce twM+lg65fSW87kE5l1oSGBiRAYQpNVYMEl6OgeY0QOEtIsocQpTwjVK6b2PnIP4ABe 8OdVgQwObZUQSpvxfjQuCFIFxPsLTWmUV8XZFEzGeOnSj+tL9r4nZQVNVg/Jk4CfaC SxcayeSWYlewxPE3eAEHOIk1l1ZesUuodm+pnIXsV2I+ga+AMly5vzY0EhodaQ50nv L9L8s6WcDMYlw== Received: from sofa.misterjones.org ([185.219.108.64] helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1r3dFR-00Dh3r-IV; Thu, 16 Nov 2023 14:21:37 +0000 Date: Thu, 16 Nov 2023 14:21:37 +0000 Message-ID: <8634x522z2.wl-maz@kernel.org> From: Marc Zyngier To: Cornelia Huck Cc: Jianyong Wu , james.morse@arm.com, will@kernel.org, rmk@armlinux.org.uk, salil.mehta@huawei.com, suzuki.poulose@arm.com, oliver.upton@linux.dev, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, justin.he@arm.com Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter In-Reply-To: <877cmhq1zp.fsf@redhat.com> References: <20231116114152.912344-1-jianyong.wu@arm.com> <877cmhq1zp.fsf@redhat.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/29.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: cohuck@redhat.com, jianyong.wu@arm.com, james.morse@arm.com, will@kernel.org, rmk@armlinux.org.uk, salil.mehta@huawei.com, suzuki.poulose@arm.com, oliver.upton@linux.dev, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, justin.he@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 16 Nov 2023 13:08:58 +0000, Cornelia Huck wrote: > > On Thu, Nov 16 2023, Jianyong Wu wrote: > > > 821d935c87b introduces support for userspace SMCCC filtering, but lack > > of a way to tell userspace if we have this feature. Add a corresponding > > feature extension can resolve this issue. > > > > For example, the incoming feature Vcpu Hotplug needs the SMCCC filter. > > As there is no way to check this feature, VMM will run into error when > > it calls this feature on an old kernel. It's bad for backward compatible. > > Can't you simply query via KVM_HAS_DEVICE_ATTR whether the SMCCC > filtering controls exist? Quite. Commit e0fc6b21616dd introduced it for that exact purpose, specifically to prevent adding more of these capabilities when there is a corresponding attribute that can be readily queried. M. -- Without deviation from the norm, progress is not possible.