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

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Message-ID: <20200120165640-mutt-send-email-mst@kernel.org>
Date:   Mon, 20 Jan 2020 16:59:40 -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 09:47:18PM +0000, Shahaf Shuler wrote:
> Monday, January 20, 2020 11:25 PM, Michael S. Tsirkin:
> > 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...
> 
> The fact memory hotplug is enabled doesn't necessarily means there is not cold-plugged memory on the hot plugged slots. 
> So your claim is majority of guests are deployed w/o any cold-plugged memory? 

Sorry for not being clear. I was merely saying that dynamic one
can't be optional, and static one can. So how about we
start just with the dynamic one, then add the static one
as a later optimization?


> > 
> > > >
> > > > > > 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