From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756035AbeDXFno (ORCPT ); Tue, 24 Apr 2018 01:43:44 -0400 Received: from mail-lf0-f67.google.com ([209.85.215.67]:47072 "EHLO mail-lf0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751469AbeDXFnl (ORCPT ); Tue, 24 Apr 2018 01:43:41 -0400 X-Google-Smtp-Source: AB8JxZoTSy+M6ZsMfVkePyAxfLi7HwkPdEcuRSV/DK3M0eJwCwOMJ2rS7w+e4npzAT/NSPcNwoNADA== Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver To: Boris Ostrovsky , Wei Liu Cc: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , jgross@suse.com, Artem Mygaiev , Dongwon Kim , konrad.wilk@oracle.com, airlied@linux.ie, "Oleksandr_Andrushchenko@epam.com" , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, "Potrola, MateuszX" , xen-devel@lists.xenproject.org, daniel.vetter@intel.com References: <5d8fec7f-956c-378f-be90-f45029385740@gmail.com> <20180416192905.GA18096@downor-Z87X-UD5H> <20180417075928.GT31310@phenom.ffwll.local> <20180417205744.GA15930@downor-Z87X-UD5H> <41487acb-a67a-8933-d0c3-702c19b0938e@gmail.com> <20180418073508.ptvntwedczpvl7bx@MacBook-Pro-de-Roger.local> <20180418101058.hyqk3gr3b2ibxswu@MacBook-Pro-de-Roger.local> <20180420071914.GG31310@phenom.ffwll.local> <76cdc65a-7bb1-9377-7bc5-6164e32f7b5d@gmail.com> <20180423115242.ywdwqblj2aseu3fr@citrix.com> <61105351-8896-072b-abf0-757c7f6c0edf@gmail.com> From: Oleksandr Andrushchenko Message-ID: <30ddb005-96c8-f47c-2e23-4e0d354e5842@gmail.com> Date: Tue, 24 Apr 2018 08:43:37 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/24/2018 01:41 AM, Boris Ostrovsky wrote: > On 04/23/2018 08:10 AM, Oleksandr Andrushchenko wrote: >> On 04/23/2018 02:52 PM, Wei Liu wrote: >>> On Fri, Apr 20, 2018 at 02:25:20PM +0300, Oleksandr Andrushchenko wrote: >>>>>>      the gntdev. >>>>>> >>>>>> I think this is generic enough that it could be implemented by a >>>>>> device not tied to Xen. AFAICT the hyper_dma guys also wanted >>>>>> something similar to this. >>>>> You can't just wrap random userspace memory into a dma-buf. We've >>>>> just had >>>>> this discussion with kvm/qemu folks, who proposed just that, and >>>>> after a >>>>> bit of discussion they'll now try to have a driver which just wraps a >>>>> memfd into a dma-buf. >>>> So, we have to decide either we introduce a new driver >>>> (say, under drivers/xen/xen-dma-buf) or extend the existing >>>> gntdev/balloon to support dma-buf use-cases. >>>> >>>> Can anybody from Xen community express their preference here? >>>> >>> Oleksandr talked to me on IRC about this, he said a few IOCTLs need to >>> be added to either existing drivers or a new driver. >>> >>> I went through this thread twice and skimmed through the relevant >>> documents, but I couldn't see any obvious pros and cons for either >>> approach. So I don't really have an opinion on this. >>> >>> But, assuming if implemented in existing drivers, those IOCTLs need to >>> be added to different drivers, which means userspace program needs to >>> write more code and get more handles, it would be slightly better to >>> implement a new driver from that perspective. >> If gntdev/balloon extension is still considered: >> >> All the IOCTLs will be in gntdev driver (in current xen-zcopy >> terminology): >>  - DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS >>  - DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS >>  - DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE >> >> Balloon driver extension, which is needed for contiguous/DMA >> buffers, will be to provide new *kernel API*, no UAPI is needed. >> > > So I am obviously a bit late to this thread, but why do you need to add > new ioctls to gntdev and balloon? Doesn't this driver manage to do what > you want without any extensions? 1. I only (may) need to add IOCTLs to gntdev 2. balloon driver needs to be extended, so it can allocate contiguous (DMA) memory, not IOCTLs/UAPI here, all lives in the kernel. 3. The reason I need to extend gnttab with new IOCTLs is to provide new functionality to create a dma-buf from grant references and to produce grant references for a dma-buf. This is what I have as UAPI description for xen-zcopy driver: 1. DRM_IOCTL_XEN_ZCOPY_DUMB_FROM_REFS This will create a DRM dumb buffer from grant references provided by the frontend. The intended usage is:   - Frontend     - creates a dumb/display buffer and allocates memory     - grants foreign access to the buffer pages     - passes granted references to the backend   - Backend     - issues DRM_XEN_ZCOPY_DUMB_FROM_REFS ioctl to map       granted references and create a dumb buffer     - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD     - requests real HW driver/consumer to import the PRIME buffer with       DRM_IOCTL_PRIME_FD_TO_HANDLE     - uses handle returned by the real HW driver   - at the end:     o closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE     o closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE     o closes file descriptor of the exported buffer 2. DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS This will grant references to a dumb/display buffer's memory provided by the backend. The intended usage is:   - Frontend     - requests backend to allocate dumb/display buffer and grant references       to its pages   - Backend     - requests real HW driver to create a dumb with DRM_IOCTL_MODE_CREATE_DUMB     - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD     - requests zero-copy driver to import the PRIME buffer with       DRM_IOCTL_PRIME_FD_TO_HANDLE     - issues DRM_XEN_ZCOPY_DUMB_TO_REFS ioctl to       grant references to the buffer's memory.     - passes grant references to the frontend  - at the end:     - closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE     - closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE     - closes file descriptor of the imported buffer 3. DRM_XEN_ZCOPY_DUMB_WAIT_FREE This will block until the dumb buffer with the wait handle provided be freed: this is needed for synchronization between frontend and backend in case frontend provides grant references of the buffer via DRM_XEN_ZCOPY_DUMB_FROM_REFS IOCTL and which must be released before backend replies with XENDISPL_OP_DBUF_DESTROY response. wait_handle must be the same value returned while calling DRM_XEN_ZCOPY_DUMB_FROM_REFS IOCTL. So, as you can see the above functionality is not covered by the existing UAPI of the gntdev driver. Now, if we change dumb -> dma-buf and remove DRM code (which is only a wrapper here on top of dma-buf) we get new driver for dma-buf for Xen. This is why I have 2 options here: either create a dedicated driver for this (e.g. re-work xen-zcopy to be DRM independent and put it under drivers/xen/xen-dma-buf, for example) or extend the existing gntdev driver with the above UAPI + make changes to the balloon driver to provide kernel API for DMA buffer allocations. So, this is where I need to understand Xen community's preference, so the implementation is not questioned later on. > -boris Thank you, Oleksandr