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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5708AC7618F for ; Mon, 15 Jul 2019 14:38:51 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 313DA20651 for ; Mon, 15 Jul 2019 14:38:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="VbAH0KUX" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 313DA20651 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender:Content-Type: Content-Transfer-Encoding:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=n9LYbK+r1EpNiewNcLxDWvf8Bhxm8gWs76rOuqWTYeE=; b=VbAH0KUX5FSoUIUO8yMDxOo12 XXF43KxHi0LIV/vHpoPSHTcvPL5r56d4bYcRXqW48LSjQjOVgqcjkIK55jPHTSB7HbnjKH7lzHyoH FzrJUaj04Dq/4P5At9F7CUYDBJXcWjBU2ijJGH3X6wpVvugQAMv2++G4ufvSyQI75argEKzMfpnP5 N3eztCyXeVmSVfD+vcfkYmBZS+yenFIuXoIXhuDFmuLsuK5z1bdVSfvWU33KKd9qMHxa6kuLerF9E 2r05faNIKKmHR/waq29nlXyFUiAX3mAmphFzUDwovPdVi5aUsn8WN+SCBtH70u/pvwQZYHWx+juQM DXwqBvVXQ==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hn27t-0007Hg-RM; Mon, 15 Jul 2019 14:38:49 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hn27q-0007Eu-Ae for linux-arm-kernel@lists.infradead.org; Mon, 15 Jul 2019 14:38:47 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C80BF28; Mon, 15 Jul 2019 07:38:44 -0700 (PDT) Received: from [10.1.196.50] (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A1A613F59C; Mon, 15 Jul 2019 07:38:43 -0700 (PDT) Subject: Re: [RFC v2 12/14] arm64/lib: asid: Allow user to update the context under the lock To: James Morse References: <20190620130608.17230-1-julien.grall@arm.com> <20190620130608.17230-13-julien.grall@arm.com> From: Julien Grall Message-ID: <446cfa1a-71be-3ae2-4107-02dd0f164843@arm.com> Date: Mon, 15 Jul 2019 15:38:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190715_073846_427300_84C95644 X-CRM114-Status: GOOD ( 17.09 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: julien.thierry@arm.com, marc.zyngier@arm.com, catalin.marinas@arm.com, suzuki.poulose@arm.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 03/07/2019 18:35, James Morse wrote: > Hi Julien, Hi James, > On 20/06/2019 14:06, Julien Grall wrote: >> Some users of the ASID allocator (e.g VMID) will require to update the >> context when a new ASID is generated. This has to be protected by a lock >> to prevent concurrent modification. >> >> Rather than introducing yet another lock, it is possible to re-use the >> allocator lock for that purpose. This patch introduces a new callback >> that will be call when updating the context. > > You're using this later in the series to mask out the generation from the atomic64 to > leave just the vmid. You are right. > > Where does this concurrent modification happen? The value is only written if we have a > rollover, and while its active the only bits that could change are the generation. > (subsequent vCPUs that take the slow path for the same VM will see the updated generation > and skip the new_context call) > > If we did the generation filtering in update_vmid() after the call to > asid_check_context(), what would go wrong? > It happens more often than is necessary and would need a WRITE_ONCE(), but the vmid can't > change until we become preemptible and another vCPU gets a chance to make its vmid active. I think I was over cautious. Pre-filtering after asid_check_context() is equally fine as long as update_vttbr() is called from preemptible context. Cheers, -- Julien Grall _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel