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

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Date:   Mon, 20 Jan 2020 16:48:55 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Jason Wang <jasowang@...hat.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 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