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

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Date:   Tue, 3 Sep 2019 16:32:45 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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

Hi, Christoph,

On 9/3/19 3:46 PM, Christoph Hellwig wrote:
> On Tue, Sep 03, 2019 at 03:15:01PM +0200, Thomas Hellström (VMware) wrote:
>> From: Thomas Hellstrom <thellstrom@...are.com>
>>
>> The force_dma_unencrypted symbol is needed by TTM to set up the correct
>> page protection when memory encryption is active. Export it.
> NAK.  This is a helper for the core DMA code and drivers have no
> business looking at it.

Is this a layer violation concern, that is, would you be ok with a 
similar helper for TTM, or is it that you want to force the graphics 
drivers into adhering strictly to the DMA api, even when it from an 
engineering perspective makes no sense?

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?

Thanks,

Thomas