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Date: Tue, 21 Jan 2020 00:47:15 -0500 From: "Michael S. Tsirkin" <mst@...hat.com> To: Jason Wang <jasowang@...hat.com> Cc: Jason Gunthorpe <jgg@...lanox.com>, Shahaf Shuler <shahafs@...lanox.com>, Rob Miller <rob.miller@...adcom.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>, Netdev <netdev@...r.kernel.org>, "Bie, Tiwei" <tiwei.bie@...el.com>, "maxime.coquelin@...hat.com" <maxime.coquelin@...hat.com>, "Liang, Cunming" <cunming.liang@...el.com>, "Wang, Zhihong" <zhihong.wang@...el.com>, "Wang, Xiao W" <xiao.w.wang@...el.com>, "haotian.wang@...ive.com" <haotian.wang@...ive.com>, "Zhu, Lingshan" <lingshan.zhu@...el.com>, "eperezma@...hat.com" <eperezma@...hat.com>, "lulu@...hat.com" <lulu@...hat.com>, Parav Pandit <parav@...lanox.com>, "Tian, Kevin" <kevin.tian@...el.com>, "stefanha@...hat.com" <stefanha@...hat.com>, "rdunlap@...radead.org" <rdunlap@...radead.org>, "hch@...radead.org" <hch@...radead.org>, Ariel Adam <aadam@...hat.com>, "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>, Jiri Pirko <jiri@...lanox.com>, "hanand@...inx.com" <hanand@...inx.com>, "mhabets@...arflare.com" <mhabets@...arflare.com> Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus 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
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