From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus Date: Mon, 20 Jan 2020 16:48:55 -0500 Message-ID: <20200120164756-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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200120174933.GB3891@mellanox.com> Sender: kvm-owner@vger.kernel.org To: Jason Gunthorpe Cc: Jason Wang , 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 List-Id: virtualization@lists.linuxfoundation.org On Mon, Jan 20, 2020 at 05:49:39PM +0000, 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. > > 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 I think for starters we can assume this doesn't happen, so any change doesn't affect any buffers in use. Certainly true e.g. for memory hotplug. -- MST