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 76091CAC582 for ; Fri, 12 Sep 2025 17:16:25 +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:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: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=0gDeU8AQGQoOZmKSbYt/s3y49ykek5V3bTsRDPV3BOM=; b=1aTI4aoirOWjRWlCAAHMGsA7Zt vO1TEuNroaKiPvqP5nNcLc2zj9hOgq9auzCgewSYN7tYOpxrPr39k1atAUvQ4WGVqV/W8X6gyd6ud O4jkIcDvJhejTo1kptwL3OCWOWruZ33atxhpoY6kGcH9fjAS2+ju+DPLDFsj4rQejvWo3vSicumIW 1kZ5RFAyVW4WW6td4suuczFfSyMprSWb35N2yTniGAFf9wqWtmCz4Ylvh9VedyFEcegQywbfsT3HF gDjHnFKgvl+D8BNmRTqSfFOLbi7b7VBqx5J+JXT3T73Sb5uzR+55+PGBAfl7AqyIQyA1VQK2Pmkph oYw3vf1w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1ux7Nj-0000000AjSX-2Mj9; Fri, 12 Sep 2025 17:16:19 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1ux7Nh-0000000AjR6-0Grn for linux-arm-kernel@lists.infradead.org; Fri, 12 Sep 2025 17:16:18 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id CC358409FB; Fri, 12 Sep 2025 17:16:14 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 25BC4C4CEF1; Fri, 12 Sep 2025 17:16:11 +0000 (UTC) Date: Fri, 12 Sep 2025 18:16:09 +0100 From: Catalin Marinas To: Will Deacon Cc: Yeoreum Yun , broonie@kernel.org, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, james.morse@arm.com, ardb@kernel.org, scott@os.amperecomputing.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND v7 6/6] arm64: futex: support futex with FEAT_LSUI Message-ID: References: <20250816151929.197589-1-yeoreum.yun@arm.com> <20250816151929.197589-7-yeoreum.yun@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250912_101617_123166_7C8BC0E1 X-CRM114-Status: GOOD ( 17.52 ) 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 On Thu, Sep 11, 2025 at 04:22:24PM +0100, Will Deacon wrote: > On Sat, Aug 16, 2025 at 04:19:29PM +0100, Yeoreum Yun wrote: > > +static __always_inline int > > +__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval) > > +{ > > + unsigned int loops = LLSC_MAX_LOOPS; > > + int ret, oldval, tmp; > > + > > + uaccess_ttbr0_enable(); > > + /* > > + * there are no ldteor/stteor instructions... > > + */ > > *sigh* > > Were these new instructions not added with futex in mind? I guess it was _most_ of the futex. > I wonder whether CAS would be better than exclusives for xor... I was first thinking we could share some of the code with __futex_cmpxchg() but... > > +static __always_inline int > > +__lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval) > > +{ > > + int ret = 0; > > + unsigned int loops = LLSC_MAX_LOOPS; > > + u32 val, tmp; > > + > > + uaccess_ttbr0_enable(); > > + /* > > + * cas{al}t doesn't support word size... > > + */ > > What about just aligning down and doing a 64-bit cas in that case? I think it gets more complicated. Here we get the oldval from the caller, so no need to do a read. With CAS, we'd need to read the full 64-bit, replace half of it with oldval and newval just to be able to do the operation. On top of this, we need to check which half of the 64-bit value. I think it to hairy for little benefit. -- Catalin