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:   Tue, 21 Jan 2020 09:17:45 -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: <20200121091636-mutt-send-email-mst@kernel.org>
References: <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>
 <2a324cec-2863-58f4-c58a-2414ee32c930@redhat.com>
 <20200121004047-mutt-send-email-mst@kernel.org>
 <b9ad744e-c4cd-82f9-f56a-1ecc185e9cd7@redhat.com>
 <20200121031506-mutt-send-email-mst@kernel.org>
 <20200121140456.GA12330@mellanox.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200121140456.GA12330@mellanox.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jan 21, 2020 at 02:05:04PM +0000, Jason Gunthorpe wrote:
> On Tue, Jan 21, 2020 at 03:15:43AM -0500, Michael S. Tsirkin wrote:
> > > This sounds more flexible e.g driver may choose to implement static mapping
> > > one through commit. But a question here, it looks to me this still requires
> > > the DMA to be synced with at least commit here. Otherwise device may get DMA
> > > fault? Or device is expected to be paused DMA during begin?
> > > 
> > > Thanks
> > 
> > For example, commit might switch one set of tables for another,
> > without need to pause DMA.
> 
> I'm not aware of any hardware that can do something like this
> completely atomically..

FWIW VTD can do this atomically.

> Any mapping change API has to be based around add/remove regions
> without any active DMA (ie active DMA is a guest error the guest can
> be crashed if it does this)
> 
> Jason

Right, lots of cases are well served by only changing parts of
mapping that aren't in active use. Memory hotplug is such a case.
That's not the same as a completely static mapping.

-- 
MST