Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 20 Jan 2020 16:48:55 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        Shahaf Shuler <shahafs@mellanox.com>,
        Rob Miller <rob.miller@broadcom.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Netdev <netdev@vger.kernel.org>,
        "Bie, Tiwei" <tiwei.bie@intel.com>,
        "maxime.coquelin@redhat.com" <maxime.coquelin@redhat.com>,
        "Liang, Cunming" <cunming.liang@intel.com>,
        "Wang, Zhihong" <zhihong.wang@intel.com>,
        "Wang, Xiao W" <xiao.w.wang@intel.com>,
        "haotian.wang@sifive.com" <haotian.wang@sifive.com>,
        "Zhu, Lingshan" <lingshan.zhu@intel.com>,
        "eperezma@redhat.com" <eperezma@redhat.com>,
        "lulu@redhat.com" <lulu@redhat.com>,
        Parav Pandit <parav@mellanox.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        "stefanha@redhat.com" <stefanha@redhat.com>,
        "rdunlap@infradead.org" <rdunlap@infradead.org>,
        "hch@infradead.org" <hch@infradead.org>,
        Ariel Adam <aadam@redhat.com>,
        "jakub.kicinski@netronome.com" <jakub.kicinski@netronome.com>,
        Jiri Pirko <jiri@mellanox.com>,
        "hanand@xilinx.com" <hanand@xilinx.com>,
        "mhabets@solarflare.com" <mhabets@solarflare.com>
Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
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>
 <CAJPjb1+fG9L3=iKbV4Vn13VwaeDZZdcfBPvarogF_Nzhk+FnKg@mail.gmail.com>
 <AM0PR0502MB379553984D0D55FDE25426F6C3330@AM0PR0502MB3795.eurprd05.prod.outlook.com>
 <d69918ca-8af4-44b2-9652-633530d4c113@redhat.com>
 <20200120174933.GB3891@mellanox.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200120174933.GB3891@mellanox.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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