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 22A4AC433F5 for ; Thu, 21 Apr 2022 13:25:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352992AbiDUN2X (ORCPT ); Thu, 21 Apr 2022 09:28:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47348 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348045AbiDUN2U (ORCPT ); Thu, 21 Apr 2022 09:28:20 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EF4C0E016 for ; Thu, 21 Apr 2022 06:25:30 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 8CCDEB82488 for ; Thu, 21 Apr 2022 13:25:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6746C385A5; Thu, 21 Apr 2022 13:25:25 +0000 (UTC) Date: Thu, 21 Apr 2022 14:25:22 +0100 From: Catalin Marinas To: Arnd Bergmann Cc: Christoph Hellwig , Ard Biesheuvel , Herbert Xu , Will Deacon , Marc Zyngier , Greg Kroah-Hartman , Andrew Morton , Linus Torvalds , Linux Memory Management List , Linux ARM , Linux Kernel Mailing List , "David S. Miller" Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 21, 2022 at 02:28:45PM +0200, Arnd Bergmann wrote: > On Thu, Apr 21, 2022 at 1:06 PM Catalin Marinas wrote: > > On Thu, Apr 21, 2022 at 12:20:22AM -0700, Christoph Hellwig wrote: > > > Btw, there is another option: Most real systems already require having > > > swiotlb to bounce buffer in some cases. We could simply force bounce > > > buffering in the dma mapping code for too small or not properly aligned > > > transfers and just decrease the dma alignment. > > > > We can force bounce if size is small but checking the alignment is > > trickier. Normally the beginning of the buffer is aligned but the end is > > at some sizeof() distance. We need to know whether the end is in a > > kmalloc-128 cache and that requires reaching out to the slab internals. > > That's doable and not expensive but it needs to be done for every small > > size getting to the DMA API, something like (for mm/slub.c): > > > > folio = virt_to_folio(x); > > slab = folio_slab(folio); > > if (slab->slab_cache->align < ARCH_DMA_MINALIGN) > > ... bounce ... > > > > (and a bit different for mm/slab.c) > > I think the decision to bounce or not can be based on the actual > cache line size at runtime, so most commonly 64 bytes on arm64, > even though the compile-time limit is 128 bytes. > > We also know that larger slabs are all cacheline aligned, so simply > comparing the transfer size is enough to rule out most, in this case > any transfer larger than 96 bytes must come from the kmalloc-128 > or larger cache, so that works like before. There's also the case with 128-byte cache lines and kmalloc-192. > For transfers <=96 bytes, the possibilities are: > > 1.kmalloc-32 or smaller, always needs to bounce > 2. kmalloc-96, but at least one byte in partial cache line, > need to bounce > 3. kmalloc-64, may skip the bounce. > 4. kmalloc-128 or larger, or not a slab cache but a partial > transfer, may skip the bounce. > > I would guess that the first case is the most common here, > so unless bouncing one or two cache lines is extremely > expensive, I don't expect it to be worth optimizing for the latter > two cases. I think so. If someone complains of a performance regression, we can look at optimising the bounce. I have a suspicion the cost of copying two cache lines is small compared to swiotlb_find_slots() etc. -- Catalin