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 mm01.cs.columbia.edu (mm01.cs.columbia.edu [128.59.11.253]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D74DC433F5 for ; Tue, 23 Nov 2021 10:11:40 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 93E394B0BE; Tue, 23 Nov 2021 05:11:40 -0500 (EST) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id VVh-XUHQTPk5; Tue, 23 Nov 2021 05:11:39 -0500 (EST) Received: from mm01.cs.columbia.edu (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 504F64B0D0; Tue, 23 Nov 2021 05:11:39 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 268364B0BE for ; Tue, 23 Nov 2021 05:11:38 -0500 (EST) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id nFyAiabcwqDb for ; Tue, 23 Nov 2021 05:11:37 -0500 (EST) Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by mm01.cs.columbia.edu (Postfix) with ESMTPS id E194A4B08E for ; Tue, 23 Nov 2021 05:11:36 -0500 (EST) 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 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") 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 Cc: kernel-team@android.com, kvm@vger.kernel.org, Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu 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. _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm