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Date:   Tue, 27 Oct 2020 08:58:10 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Christoph Hellwig <hch@infradead.org>
CC:     Tom Lendacky <thomas.lendacky@amd.com>, <x86@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-arch@vger.kernel.org>,
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        Brijesh Singh <brijesh.singh@amd.com>
Subject: Re: AMD SME encrpytion and PCI BAR pages to user space
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On Tue, Oct 27, 2020 at 08:43:57AM +0000, Christoph Hellwig wrote:
> On Wed, Oct 21, 2020 at 01:03:22PM -0300, Jason Gunthorpe wrote:
> > Oh, interesting.. Yes the issue is no userspace DMA stuff uses the DMA
> > API correctly (because it is in userspace)
> > 
> > So SWIOTLB tricks don't work, I wish the dma_map could fail for these
> > situations
> 
> Userspace DMA by definition also does not use dma_map..

? Sure it does, ib_dma_map_sg_attrs() is what RDMA uses

What all the userspace users skip is the dma_sync*() - that would
require a kernel call which defeats the point.

So, my desire is some flag to dma_map_sg() that says
  'user space mapping no dma_sync_*'

ie dma_sync_* is a NOP

Then things like SWIOTLB on the SEV system can fail with an error code
instead of malfunctioning

If FOLL_LONGERM is some estimate of this pattern then we have these users:
 - drivers/infiniband
 - v4l
 - vdpa
 - xdp
 - rds
 - habana labs

Jason