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

Andy Lutomirski luto at amacapital.net
Tue Sep 3 23:15:47 UTC 2019

> On Sep 3, 2019, at 3:15 PM, Thomas Hellström (VMware) <thomas_os at shipmail.org> wrote:
>> On 9/4/19 12:08 AM, Thomas Hellström (VMware) wrote:
>>> On 9/3/19 11:46 PM, Andy Lutomirski wrote:
>>> On Tue, Sep 3, 2019 at 2:05 PM Thomas Hellström (VMware)
>>> <thomas_os at shipmail.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.
>> Yes, so in this case we use dma_alloc_coherent().
>> With SEV, that gives us unencrypted pages. (Pages whose linear kernel map is marked unencrypted). With SME that (typcially) gives us encrypted pages. In both these cases, vm_get_page_prot() returns
>> an encrypted page protection, which lands in vma->vm_page_prot.
>> In the SEV case, we therefore need to modify the page protection to unencrypted. Hence we need to know whether we're running under SEV and therefore need to modify the protection. If not, the user-space PTE would incorrectly have the encryption flag set.

I’m still confused. You got unencrypted pages with an unencrypted PFN. Why do you need to fiddle?  You have a PFN, and you’re inserting it with vmf_insert_pfn().  This should just work, no?  There doesn’t seem to be any real funny business in dma_mmap_attrs() or dma_common_mmap().

But, reading this, I have more questions:

Can’t you get rid of cvma by using vmf_insert_pfn_prot()?

Would it make sense to add a vmf_insert_dma_page() to directly do exactly what you’re trying to do?

And a broader question just because I’m still confused: why isn’t the encryption bit in the PFN?  The whole SEV/SME system seems like it’s trying a bit to hard to be fully invisible to the kernel.

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