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Date:   Thu, 17 Jan 2019 10:30:01 +0100
From:   "hch@lst.de" <hch@lst.de>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     "hch@lst.de" <hch@lst.de>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        "daniel.vetter@ffwll.ch" <daniel.vetter@ffwll.ch>,
        "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
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Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator
Message-ID: <20190117093001.GB31303@lst.de>
References: <20190104223531.GA1705@ziepe.ca> <20190110234218.GM6890@ziepe.ca> <20190114094856.GB29604@lst.de> <1fb20ab4b171b281e9994b6c55734c120958530b.camel@vmware.com> <20190115212501.GE22045@ziepe.ca> <20190116161134.GA29041@lst.de> <20190116172436.GM22045@ziepe.ca>
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On Wed, Jan 16, 2019 at 10:24:36AM -0700, Jason Gunthorpe wrote:
> The fact is there is 0 industry interest in using RDMA on platforms
> that can't do HW DMA cache coherency - the kernel syscalls required to
> do the cache flushing on the IO path would just destroy performance to
> the point of making RDMA pointless. Better to use netdev on those
> platforms.

In general there is no syscall required for doing cache flushing, you
just issue the proper instructions directly from userspace. 

> The reality is that *all* the subsytems doing DMA kernel bypass are
> ignoring the DMA mapping rules, I think we should support this better,
> and just accept that user space DMA will not be using syncing. Block
> access in cases when this is required, otherwise let it work as is
> today.

In that case we just need to block userspace DMA access entirely.
Which given the amount of problems it creates sounds like a pretty
good idea anyway.