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

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Date:   Mon, 20 Jan 2020 20:51:43 +0000
From:   Shahaf Shuler <shahafs@...lanox.com>
To:     Jason Gunthorpe <jgg@...lanox.com>,
        Jason Wang <jasowang@...hat.com>
CC:     Rob Miller <rob.miller@...adcom.com>,
        "Michael S. Tsirkin" <mst@...hat.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

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. 

> 
> > > 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