netdev - Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce

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Message-ID: <20181119192512.GD4593@redhat.com>
Date:   Mon, 19 Nov 2018 14:25:13 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
        Kenneth Lee <liguozhu@...ilicon.com>,
        Tim Sell <timothy.sell@...sys.com>, linux-doc@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Zaibo Xu <xuzaibo@...wei.com>, zhangfei.gao@...mail.com,
        linuxarm@...wei.com, haojian.zhuang@...aro.org,
        Hao Fang <fanghao11@...wei.com>,
        Gavin Schenk <g.schenk@...elmann.de>,
        RDMA mailing list <linux-rdma@...r.kernel.org>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Doug Ledford <dledford@...hat.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        David Kershner <david.kershner@...sys.com>,
        Kenneth Lee <nek.in.cn@...il.com>,
        Johan Hovold <johan@...nel.org>,
        Cyrille Pitchen <cyrille.pitchen@...e-electrons.com>,
        Sagar Dharia <sdharia@...eaurora.org>,
        Jens Axboe <axboe@...nel.dk>, guodong.xu@...aro.org,
        linux-netdev <netdev@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, Vinod Koul <vkoul@...nel.org>,
        linux-crypto@...r.kernel.org,
        Philippe Ombredanne <pombredanne@...b.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-accelerators@...ts.ozlabs.org
Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce

On Mon, Nov 19, 2018 at 07:19:04PM +0000, Christopher Lameter wrote:
> On Mon, 19 Nov 2018, Jerome Glisse wrote:
> 
> > > IIRC this is solved in IB by automatically calling
> > > madvise(MADV_DONTFORK) before creating the MR.
> > >
> > > MADV_DONTFORK
> > >   .. This is useful to prevent copy-on-write semantics from changing the
> > >   physical location of a page if the parent writes to it after a
> > >   fork(2) ..
> >
> > This would work around the issue but this is not transparent ie
> > range marked with DONTFORK no longer behave as expected from the
> > application point of view.
> 
> Why would anyone expect a range registered via MR behave as normal? Device
> I/O is going on into that range. Memory is already special.
> 
> 
> > Also it relies on userspace doing the right thing (which is not
> > something i usualy trust :)).
> 
> This has been established practice for 15 or so years in a couple of use
> cases. Again user space already has to be doing special things in order to
> handle RDMA is that area.

Yes RDMA as an existing historical track record and thus people
should now be aware of its limitation. What i am fighting against
is new addition to kernel that pretend to do SVA (share virtual
address) while their hardware is not realy doing SVA. SVA with
IOMMU and ATS/PASID is fine, SVA in software with device driver
that abide to mmu notifier is fine. Anything else is not.

So i am just worrying about new user and making sure they under-
stand what is happening and not sell to their user something
false.

Cheers,
Jérôme