linux-kernel - Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD memory encryption

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Date:   Tue, 3 Sep 2019 14:46:27 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        pv-drivers@...are.com,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD
 memory encryption

On Tue, Sep 3, 2019 at 2:05 PM Thomas Hellström (VMware)
<thomas_os@...pmail.org> wrote:
>
> On 9/3/19 10:51 PM, Dave Hansen wrote:
> > On 9/3/19 1:36 PM, Thomas Hellström (VMware) wrote:
> >> So the question here should really be, can we determine already at mmap
> >> 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.