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 2BA60C433F5 for ; Tue, 23 Nov 2021 10:11:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235345AbhKWKOo (ORCPT ); Tue, 23 Nov 2021 05:14:44 -0500 Received: from mail.kernel.org ([198.145.29.99]:39794 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235308AbhKWKOn (ORCPT ); Tue, 23 Nov 2021 05:14:43 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DE30860E73; Tue, 23 Nov 2021 10:11:35 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1mpSlx-007FS7-Ja; Tue, 23 Nov 2021 10:11:33 +0000 Date: Tue, 23 Nov 2021 10:11:33 +0000 Message-ID: <87mtlvchbe.wl-maz@kernel.org> From: Marc Zyngier To: Mark Brown Cc: Zenghui Yu , kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, James Morse , Suzuki K Poulose , Alexandru Elisei , Quentin Perret , Will Deacon , kernel-team@android.com Subject: Re: [PATCH v2 2/5] KVM: arm64: Get rid of host SVE tracking/saving In-Reply-To: References: <20211028111640.3663631-1-maz@kernel.org> <20211028111640.3663631-3-maz@kernel.org> <5ab3836f-2b39-2ff5-3286-8258addd01e4@huawei.com> <871r38dvyr.wl-maz@kernel.org> <87v90kcb8u.wl-maz@kernel.org> 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/27.1 (x86_64-pc-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: broonie@kernel.org, yuzenghui@huawei.com, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, james.morse@arm.com, suzuki.poulose@arm.com, alexandru.elisei@arm.com, qperret@google.com, will@kernel.org, kernel-team@android.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: kvm@vger.kernel.org On Mon, 22 Nov 2021 18:30:16 +0000, Mark Brown wrote: > > On Mon, Nov 22, 2021 at 06:10:25PM +0000, Marc Zyngier wrote: > > Mark Brown wrote: > > > > While we're on the subject of potential future work we might in future > > > want to not disable SVE on every syscall if (as seems likely) it turns > > > out that that's more performant for small vector lengths > > > How are you going to retrofit that into userspace? This would be an > > ABI change, and I'm not sure how you'd want to deal with that > > transition... > > We don't need to change the ABI, the ABI just says we zero the registers > that aren't shared with FPSIMD. Instead of doing that on taking a SVE > access trap to reenable SVE after having disabled TIF_SVE we could do > that during the syscall, userspace can't tell the difference other than > via the different formats we use to report the SVE register set via > ptrace if it single steps over a syscall. Even then I'm struggling to > think of a scenario where userspace would be relying on that. That's not the point I'm trying to make. Userspace expects to have lost SVE information over a syscall (even if the VL is 128, it expects to have lost P0..P15 and FFR). How do you plan to tell userspace that this behaviour has changed? M. -- Without deviation from the norm, progress is not possible.