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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-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2020/12/31 =E4=B8=8B=E5=8D=881:15, Yongji Xie wrote: > On Thu, Dec 31, 2020 at 10:49 AM Jason Wang wrote= : >> >> On 2020/12/30 =E4=B8=8B=E5=8D=886:12, Yongji Xie wrote: >>> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang wrot= e: >>>> On 2020/12/30 =E4=B8=8B=E5=8D=883:09, Yongji Xie wrote: >>>>> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang wr= ote: >>>>>> On 2020/12/29 =E4=B8=8B=E5=8D=886:26, 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 =E4=B8=8B=E5=8D=884:14, Yongji Xie wrote: >>>>>>>>>>>> I see. So all the above two questions are because VHOST_IOTL= B_INVALIDATE >>>>>>>>>>>> is expected to be synchronous. This need to be solved by twe= aking the >>>>>>>>>>>> current VDUSE API or we can re-visit to go with descriptors = relaying >>>>>>>>>>>> first. >>>>>>>>>>>> >>>>>>>>>>> Actually all vdpa related operations are synchronous in curre= nt >>>>>>>>>>> implementation. The ops.set_map/dma_map/dma_unmap should not = return >>>>>>>>>>> until the VDUSE_UPDATE_IOTLB/VDUSE_INVALIDATE_IOTLB message i= s 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 unma= p, 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 th= e >>>>> page fault handler. So it can't be used in coherent DMA mapping cas= e >>>>> which needs some memory with contiguous virtual addresses. I can us= e >>>>> vmalloc() to do allocation for the bounce buffer instead. But it mi= ght >>>>> 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 ca= n >>> 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 i= s >> 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=20 (alloc_pages_exact()). Or do you mean it would be a little bit hard to=20 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 >