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=-10.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 C2D61C433E0 for ; Thu, 31 Dec 2020 05:52:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9223022262 for ; Thu, 31 Dec 2020 05:52:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726555AbgLaFvt (ORCPT ); Thu, 31 Dec 2020 00:51:49 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:31669 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726218AbgLaFvt (ORCPT ); Thu, 31 Dec 2020 00:51:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1609393822; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=58HwBb3M7PAiscWQmllnEdkUdFZgFa03LEx+BxIEido=; b=LQsSSdDYux53O83ctXgwwCNdwy//UfRNBy1bj9rU+uR3X0EFF3wIE+fam2F04nA3G8HFNF Pu8P6FaLu0EjEi0jWqwfDYM7irCyRReYwnZxOK6MJOIeQaKOO8bBT2G2wZHpVnhiDrBPwD jnRlQPj/3MpFDmU0tTqVWgJWYqwAeSc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-238-cVm3mfXEO0u76e2pupugXw-1; Thu, 31 Dec 2020 00:50:18 -0500 X-MC-Unique: cVm3mfXEO0u76e2pupugXw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4EF05180A097; Thu, 31 Dec 2020 05:50:16 +0000 (UTC) Received: from [10.72.12.236] (ovpn-12-236.pek2.redhat.com [10.72.12.236]) by smtp.corp.redhat.com (Postfix) with ESMTP id A0DC46F7F4; Thu, 31 Dec 2020 05:50:00 +0000 (UTC) Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message To: Yongji Xie Cc: "Michael S. Tsirkin" , Stefan Hajnoczi , sgarzare@redhat.com, Parav Pandit , akpm@linux-foundation.org, Randy Dunlap , Matthew Wilcox , viro@zeniv.linux.org.uk, axboe@kernel.dk, bcrl@kvack.org, corbet@lwn.net, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, kvm@vger.kernel.org, linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org References: <20201222145221.711-1-xieyongji@bytedance.com> <0e6faf9c-117a-e23c-8d6d-488d0ec37412@redhat.com> <2b24398c-e6d9-14ec-2c0d-c303d528e377@redhat.com> <1356137727.40748805.1609233068675.JavaMail.zimbra@redhat.com> <3fc6a132-9fc2-c4e2-7fb1-b5a8bfb771fa@redhat.com> From: Jason Wang Message-ID: <0885385c-ae46-158d-eabf-433ef8ecf27f@redhat.com> Date: Thu, 31 Dec 2020 13:49:59 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 2020/12/31 下午1:15, Yongji Xie wrote: > On Thu, Dec 31, 2020 at 10:49 AM Jason Wang wrote: >> >> On 2020/12/30 下午6:12, Yongji Xie wrote: >>> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang wrote: >>>> On 2020/12/30 下午3:09, Yongji Xie wrote: >>>>> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang wrote: >>>>>> On 2020/12/29 下午6:26, Yongji Xie wrote: >>>>>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang wrote: >>>>>>>> ----- Original Message ----- >>>>>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang wrote: >>>>>>>>>> On 2020/12/28 下午4:14, Yongji Xie wrote: >>>>>>>>>>>> I see. So all the above two questions are because VHOST_IOTLB_INVALIDATE >>>>>>>>>>>> is expected to be synchronous. This need to be solved by tweaking the >>>>>>>>>>>> current VDUSE API or we can re-visit to go with descriptors relaying >>>>>>>>>>>> first. >>>>>>>>>>>> >>>>>>>>>>> Actually all vdpa related operations are synchronous in current >>>>>>>>>>> implementation. The ops.set_map/dma_map/dma_unmap should not return >>>>>>>>>>> until the VDUSE_UPDATE_IOTLB/VDUSE_INVALIDATE_IOTLB message is replied >>>>>>>>>>> by userspace. Could it solve this problem? >>>>>>>>>> I was thinking whether or not we need to generate IOTLB_INVALIDATE >>>>>>>>>> message to VDUSE during dma_unmap (vduse_dev_unmap_page). >>>>>>>>>> >>>>>>>>>> If we don't, we're probably fine. >>>>>>>>>> >>>>>>>>> It seems not feasible. This message will be also used in the >>>>>>>>> virtio-vdpa case to notify userspace to unmap some pages during >>>>>>>>> consistent dma unmapping. Maybe we can document it to make sure the >>>>>>>>> users can handle the message correctly. >>>>>>>> Just to make sure I understand your point. >>>>>>>> >>>>>>>> Do you mean you plan to notify the unmap of 1) streaming DMA or 2) >>>>>>>> coherent DMA? >>>>>>>> >>>>>>>> For 1) you probably need a workqueue to do that since dma unmap can >>>>>>>> be done in irq or bh context. And if usrspace does't do the unmap, it >>>>>>>> can still access the bounce buffer (if you don't zap pte)? >>>>>>>> >>>>>>> I plan to do it in the coherent DMA case. >>>>>> Any reason for treating coherent DMA differently? >>>>>> >>>>> Now the memory of the bounce buffer is allocated page by page in the >>>>> page fault handler. So it can't be used in coherent DMA mapping case >>>>> which needs some memory with contiguous virtual addresses. I can use >>>>> vmalloc() to do allocation for the bounce buffer instead. But it might >>>>> cause some memory waste. Any suggestion? >>>> I may miss something. But I don't see a relationship between the >>>> IOTLB_UNMAP and vmalloc(). >>>> >>> In the vmalloc() case, the coherent DMA page will be taken from the >>> memory allocated by vmalloc(). So IOTLB_UNMAP is not needed anymore >>> during coherent DMA unmapping because those vmalloc'ed memory which >>> has been mapped into userspace address space during initialization can >>> be reused. And userspace should not unmap the region until we destroy >>> the device. >> >> Just to make sure I understand. My understanding is that IOTLB_UNMAP is >> only needed when there's a change the mapping from IOVA to page. >> > Yes, that's true. > >> So if we stick to the mapping, e.g during dma_unmap, we just put IOVA to >> free list to be used by the next IOVA allocating. IOTLB_UNMAP could be >> avoided. >> >> So we are not limited by how the pages are actually allocated? >> > In coherent DMA cases, we need to return some memory with contiguous > kernel virtual addresses. That is the reason why we need vmalloc() > here. If we allocate the memory page by page, the corresponding kernel > virtual addresses in a contiguous IOVA range might not be contiguous. Yes, but we can do that as what has been done in the series (alloc_pages_exact()). Or do you mean it would be a little bit hard to recycle IOVA/pages here? Thanks > And in streaming DMA cases, there is no limit. So another choice is > using vmalloc'ed memory only for coherent DMA cases. > > Not sure if this is clear for you. > > Thanks, > Yongji >