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

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Message-ID: <1bec048b-8633-4f65-0657-41f65271bcce@shipmail.org>
Date:   Tue, 3 Sep 2019 20:50:20 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Dave Hansen <dave.hansen@...el.com>,
        dri-devel@...ts.freedesktop.org, pv-drivers@...are.com,
        linux-graphics-maintainer@...are.com, linux-kernel@...r.kernel.org
Cc:     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 9/3/19 5:14 PM, Dave Hansen wrote:
> On 9/3/19 6:15 AM, Thomas Hellström (VMware) wrote:
>> The force_dma_unencrypted symbol is needed by TTM to set up the correct
>> page protection when memory encryption is active. Export it.
> It would be great if this had enough background that I didn't have to
> look at patch 4 to figure out what TTM might be.
>
> Why is TTM special?  How many other drivers would have to be modified in
> a one-off fashion if we go this way?  What's the logic behind this being
> a non-GPL export?

TTM tries to abstract mapping of graphics buffer objects regardless 
where they live. Be it in pci memory or system memory. As such it needs 
to figure out the proper page protection. For example if a buffer object 
is moved from pci memory to system memory transparently to a user-space 
application, all user-space mappings need to be killed and then 
reinstated pointing to the new location, sometimes with a new page 
protection.

I try to keep away as much as possible from the non-GPL vs GPL export 
discussions. I have no strong opinion on the subject. Although since 
sev_active() is a non-GPL export, I decided to mimic that.

Thanks
Thomas