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 93B10E7F124 for ; Tue, 26 Sep 2023 20:23:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235834AbjIZUX5 (ORCPT ); Tue, 26 Sep 2023 16:23:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231564AbjIZUXy (ORCPT ); Tue, 26 Sep 2023 16:23:54 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E5BCD11D for ; Tue, 26 Sep 2023 13:23:47 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DC976C433CA; Tue, 26 Sep 2023 20:23:45 +0000 (UTC) Date: Tue, 26 Sep 2023 21:23:43 +0100 From: Catalin Marinas To: Petr Tesarik Cc: Christoph Hellwig , Marek Szyprowski , Robin Murphy , "open list:DMA MAPPING HELPERS" , open list , Roberto Sassu , Jonathan Corbet Subject: Re: [PATCH v3] swiotlb: fix the check whether a device has used software IO TLB Message-ID: References: <20230926185556.16433-1-petr@tesarici.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230926185556.16433-1-petr@tesarici.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 26, 2023 at 08:55:56PM +0200, Petr Tesarik wrote: > When CONFIG_SWIOTLB_DYNAMIC=y, devices which do not use the software IO TLB > can avoid swiotlb lookup. A flag is added by commit 1395706a1490 ("swiotlb: > search the software IO TLB only if the device makes use of it"), the flag > is correctly set, but it is then never checked. Add the actual check here. > > Note that this code is an alternative to the default pool check, not an > additional check, because: > > 1. swiotlb_find_pool() also searches the default pool; > 2. if dma_uses_io_tlb is false, the default swiotlb pool is not used. > > Tested in a KVM guest against a QEMU RAM-backed SATA disk over virtio and > *not* using software IO TLB, this patch increases IOPS by approx 2% for > 4-way parallel I/O. > > The write memory barrier in swiotlb_dyn_alloc() is not needed, because a > newly allocated pool must always be observed by swiotlb_find_slots() before > an address from that pool is passed to is_swiotlb_buffer(). > > Correctness was verified using the following litmus test: > > C swiotlb-new-pool > > (* > * Result: Never > * > * Check that a newly allocated pool is always visible when the > * corresponding swiotlb buffer is visible. > *) > > { > mem_pools = default; > } > > P0(int **mem_pools, int *pool) > { > /* add_mem_pool() */ > WRITE_ONCE(*pool, 999); > rcu_assign_pointer(*mem_pools, pool); > } > > P1(int **mem_pools, int *flag, int *buf) > { > /* swiotlb_find_slots() */ > int *r0; > int r1; > > rcu_read_lock(); > r0 = READ_ONCE(*mem_pools); > r1 = READ_ONCE(*r0); > rcu_read_unlock(); > > if (r1) { > WRITE_ONCE(*flag, 1); > smp_mb(); > } > > /* device driver (presumed) */ > WRITE_ONCE(*buf, r1); > } > > P2(int **mem_pools, int *flag, int *buf) > { > /* device driver (presumed) */ > int r0 = READ_ONCE(*buf); > > /* is_swiotlb_buffer() */ > int r1; > int *r2; > int r3; > > smp_rmb(); > r1 = READ_ONCE(*flag); > if (r1) { > /* swiotlb_find_pool() */ > rcu_read_lock(); > r2 = READ_ONCE(*mem_pools); > r3 = READ_ONCE(*r2); > rcu_read_unlock(); > } > } > > exists (2:r0<>0 /\ 2:r3=0) (* Not found. *) > > Fixes: 1395706a1490 ("swiotlb: search the software IO TLB only if the device makes use of it") > Reported-by: Jonathan Corbet > Closes: https://lore.kernel.org/linux-iommu/87a5uz3ob8.fsf@meer.lwn.net/ > Signed-off-by: Petr Tesarik I added this for v2, here it is again: Reviewed-by: Catalin Marinas