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[79.179.85.180]) by smtp.gmail.com with ESMTPSA id x3sm18835929qts.35.2020.01.20.21.47.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Jan 2020 21:47:24 -0800 (PST) Date: Tue, 21 Jan 2020 00:47:15 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: Jason Gunthorpe , Shahaf Shuler , Rob Miller , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , Netdev , "Bie, Tiwei" , "maxime.coquelin@redhat.com" , "Liang, Cunming" , "Wang, Zhihong" , "Wang, Xiao W" , "haotian.wang@sifive.com" , "Zhu, Lingshan" , "eperezma@redhat.com" , "lulu@redhat.com" , Parav Pandit , "Tian, Kevin" , "stefanha@redhat.com" , "rdunlap@infradead.org" , "hch@infradead.org" , Ariel Adam , "jakub.kicinski@netronome.com" , Jiri Pirko , "hanand@xilinx.com" , "mhabets@solarflare.com" Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus Message-ID: <20200121004047-mutt-send-email-mst@kernel.org> References: <20200116124231.20253-1-jasowang@redhat.com> <20200116124231.20253-4-jasowang@redhat.com> <20200117070324-mutt-send-email-mst@kernel.org> <239b042c-2d9e-0eec-a1ef-b03b7e2c5419@redhat.com> <20200120174933.GB3891@mellanox.com> <2a324cec-2863-58f4-c58a-2414ee32c930@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2a324cec-2863-58f4-c58a-2414ee32c930@redhat.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, Jan 21, 2020 at 12:00:57PM +0800, Jason Wang wrote: > > On 2020/1/21 上午1:49, Jason Gunthorpe wrote: > > On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote: > > > This is similar to the design of platform IOMMU part of vhost-vdpa. We > > > decide to send diffs to platform IOMMU there. If it's ok to do that in > > > driver, we can replace set_map with incremental API like map()/unmap(). > > > > > > Then driver need to maintain rbtree itself. > > I think we really need to see two modes, one where there is a fixed > > translation without dynamic vIOMMU driven changes and one that > > supports vIOMMU. > > > I think in this case, you meant the method proposed by Shahaf that sends > diffs of "fixed translation" to device? > > It would be kind of tricky to deal with the following case for example: > > old map [4G, 16G) new map [4G, 8G) > > If we do > > 1) flush [4G, 16G) > 2) add [4G, 8G) > > There could be a window between 1) and 2). > > It requires the IOMMU that can do > > 1) remove [8G, 16G) > 2) flush [8G, 16G) > 3) change [4G, 8G) > > .... Basically what I had in mind is something like qemu memory api 0. begin 1. remove [8G, 16G) 2. add [4G, 8G) 3. commit Anyway, I'm fine with a one-shot API for now, we can improve it later. > > > > There are different optimization goals in the drivers for these two > > configurations. > > > > > > If the first one, then I think memory hotplug is a heavy flow > > > > regardless. Do you think the extra cycles for the tree traverse > > > > will be visible in any way? > > > I think if the driver can pause the DMA during the time for setting up new > > > mapping, it should be fine. > > This is very tricky for any driver if the mapping change hits the > > virtio rings. :( > > > > Even a IOMMU using driver is going to have problems with that.. > > > > Jason > > > Or I wonder whether ATS/PRI can help here. E.g during I/O page fault, > driver/device can wait for the new mapping to be set and then replay the > DMA. > > Thanks > -- MST