Date: Mon, 19 Nov 2018 14:25:13 -0500
From: Jerome Glisse <jglisse@redhat.com>
To: Christopher Lameter <cl@linux.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
        Kenneth Lee <liguozhu@hisilicon.com>,
        Tim Sell <timothy.sell@unisys.com>, linux-doc@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Zaibo Xu <xuzaibo@huawei.com>, zhangfei.gao@foxmail.com,
        linuxarm@huawei.com, haojian.zhuang@linaro.org,
        Hao Fang <fanghao11@huawei.com>,
        Gavin Schenk <g.schenk@eckelmann.de>,
        RDMA mailing list <linux-rdma@vger.kernel.org>,
        Zhou Wang <wangzhou1@hisilicon.com>,
        Doug Ledford <dledford@redhat.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?=
        <u.kleine-koenig@pengutronix.de>,
        David Kershner <david.kershner@unisys.com>,
        Kenneth Lee <nek.in.cn@gmail.com>,
        Johan Hovold <johan@kernel.org>,
        Cyrille Pitchen <cyrille.pitchen@free-electrons.com>,
        Sagar Dharia <sdharia@codeaurora.org>,
        Jens Axboe <axboe@kernel.dk>, guodong.xu@linaro.org,
        linux-netdev <netdev@vger.kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-kernel@vger.kernel.org, Vinod Koul <vkoul@kernel.org>,
        linux-crypto@vger.kernel.org,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Sanyog Kale <sanyog.r.kale@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        linux-accelerators@lists.ozlabs.org
Subject: Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce
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 <20181119164853.GA4593@redhat.com>
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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,
J?r?me