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

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Message-ID: <20190904065823.GA31794@infradead.org>
Date:   Tue, 3 Sep 2019 23:58:23 -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 Tue, Sep 03, 2019 at 10:46:18PM +0200, Thomas Hellström (VMware) wrote:
> What I mean with "from an engineering perspective" is that drivers would end
> up with a non-trivial amount of code supporting purely academic cases:
> Setups where software rendering would be faster than gpu accelerated, and
> setups on platforms where the driver would never run anyway because the
> device would never be supported on that platform...

And actually work on cases you previously called academic and which now
matter to you because your employer has a suddent interest in SEV.
Academic really is in the eye of the beholder (and of those who pay
the bills).

> That is not really true. The dma API can't handle faulting of coherent pages
> which is what this series is really all about supporting also with SEV
> active. To handle the case where we move graphics buffers or send them to
> swap space while user-space have them mapped.

And the only thing we need to support the fault handler is to add an
offset to the dma_mmap_* APIs.  Which I had planned to do for Christian
(one of the few grapics developers who actually tries to play well
with the rest of the kernel instead of piling hacks over hacks like
many others) anyway, but which hasn't happened yet.

> Still, I need a way forward and my questions weren't really answered by
> this.

This is pretty demanding.  If you "need" a way forward just work with
all the relevant people instead of piling ob local hacks.