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Date:   Thu, 28 Nov 2019 16:36:46 +0100
From:   "hch@lst.de" <hch@lst.de>
To:     Thomas Hellstrom <thellstrom@vmware.com>
Cc:     "hch@lst.de" <hch@lst.de>,
        "thomas.lendacky@amd.com" <thomas.lendacky@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "christian.koenig@amd.com" <christian.koenig@amd.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH 2/2] dma-mapping: force unencryped devices are always
 addressing limited
Message-ID: <20191128153646.GA29430@lst.de>
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On Thu, Nov 28, 2019 at 08:02:16AM +0000, Thomas Hellstrom wrote:
> > We have a hard time handling that in generic code.  Do we have any
> > good use case for SWIOTLB_FORCE not that we have force_dma_unencrypted?
> > I'd love to be able to get rid of it..
> >
> IIRC the justification for it is debugging. Drivers that don't do
> syncing correctly or have incorrect assumptions of initialization of DMA
> memory will not work properly when SWIOTLB is forced. We recently found
> a vmw_pvscsi device flaw that way...

Ok. I guess debugging is reasonable.  Although that means I need
to repsin this quite a bit as I now need a callout to dma_direct.
I'll respin it in the next days.