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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 3 Sep 2019 14:46:27 -0700
Message-ID: <CALCETrVnNpPwmRddGLku9hobE7wG30_3j+QfcYxk09hZgtaYww@mail.gmail.com>
Subject: Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD
 memory encryption
To:     =?UTF-8?Q?Thomas_Hellstr=C3=B6m_=28VMware=29?= 
        <thomas_os@shipmail.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        pv-drivers@vmware.com,
        VMware Graphics <linux-graphics-maintainer@vmware.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>
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On Tue, Sep 3, 2019 at 2:05 PM Thomas Hellstr=C3=B6m (VMware)
<thomas_os@shipmail.org> wrote:
>
> On 9/3/19 10:51 PM, Dave Hansen wrote:
> > On 9/3/19 1:36 PM, Thomas Hellstr=C3=B6m (VMware) wrote:
> >> So the question here should really be, can we determine already at mma=
p
> >> time whether backing memory will be unencrypted and adjust the *real*
> >> vma->vm_page_prot under the mmap_sem?
> >>
> >> Possibly, but that requires populating the buffer with memory at mmap
> >> time rather than at first fault time.
> > I'm not connecting the dots.
> >
> > vma->vm_page_prot is used to create a VMA's PTEs regardless of if they
> > are created at mmap() or fault time.  If we establish a good
> > vma->vm_page_prot, can't we just use it forever for demand faults?
>
> With SEV I think that we could possibly establish the encryption flags
> at vma creation time. But thinking of it, it would actually break with
> SME where buffer content can be moved between encrypted system memory
> and unencrypted graphics card PCI memory behind user-space's back. That
> would imply killing all user-space encrypted PTEs and at fault time set
> up new ones pointing to unencrypted PCI memory..
>
> >
> > Or, are you concerned that if an attempt is made to demand-fault page
> > that's incompatible with vma->vm_page_prot that we have to SEGV?
> >
> >> And it still requires knowledge whether the device DMA is always
> >> unencrypted (or if SEV is active).
> > I may be getting mixed up on MKTME (the Intel memory encryption) and
> > SEV.  Is SEV supported on all memory types?  Page cache, hugetlbfs,
> > anonymous?  Or just anonymous?
>
> SEV AFAIK encrypts *all* memory except DMA memory. To do that it uses a
> SWIOTLB backed by unencrypted memory, and it also flips coherent DMA
> memory to unencrypted (which is a very slow operation and patch 4 deals
> with caching such memory).
>

I'm still lost.  You have some fancy VMA where the backing pages
change behind the application's back.  This isn't particularly novel
-- plain old anonymous memory and plain old mapped files do this too.
Can't you all the insert_pfn APIs and call it a day?  What's so
special that you need all this magic?  ISTM you should be able to
allocate memory that's addressable by the device (dma_alloc_coherent()
or whatever) and then map it into user memory just like you'd map any
other page.

I feel like I'm missing something here.