linux-kernel - Re: [PATCH v2 1/4] x86/mm: Export force_dma

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Date:   Wed, 4 Sep 2019 05:22:04 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        dri-devel@...ts.freedesktop.org, pv-drivers@...are.com,
        linux-graphics-maintainer@...are.com, linux-kernel@...r.kernel.org,
        Thomas Hellstrom <thellstrom@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 1/4] x86/mm: Export force_dma_unencrypted

On Wed, Sep 04, 2019 at 09:32:30AM +0200, Thomas Hellström (VMware) wrote:
> That sounds great. Is there anything I can do to help out? I thought this
> was more or less a dead end since the current dma_mmap_ API requires the
> mmap_sem to be held in write mode (modifying the vma->vm_flags) whereas
> fault() only offers read mode. But that would definitely work.

We'll just need to split into a setup and faul phase.  I have some
sketches from a while ago, let me dust them off so that you can
try them.

> "If it's the latter, then I would like to reiterate that it would be better
> that we work to come up with a long term plan to add what's missing to the
> DMA api to help graphics drivers use coherent memory?"

I don't think we need a long term plan.  We've been adding features
on an as-needed basis.  And now that we have siginificanty less
implementations of the API this actually becomes much easier as well.