netdev - Re: [PATCH 3/5] vDPA: introduce vDPA bus

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Date:   Mon, 20 Jan 2020 16:25:23 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Shahaf Shuler <shahafs@...lanox.com>
Cc:     Jason Gunthorpe <jgg@...lanox.com>,
        Jason Wang <jasowang@...hat.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 Mon, Jan 20, 2020 at 08:51:43PM +0000, Shahaf Shuler wrote:
> Monday, January 20, 2020 7:50 PM, Jason Gunthorpe:
> > Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
> > 
> > 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.
> 
> +1.
> It will be best to have one API for static config (i.e. mapping can be
> set only before virtio device gets active), and one API for dynamic
> changes that can be set after the virtio device is active. 

Frankly I don't see when we'd use the static one.
Memory hotplug is enabled for most guests...

> > 
> > > > 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