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

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

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@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.
>
> /Thomas
>
>
And, of course, had we not been "fancy", we could have used 
dma_mmap_coherent(), which in theory should set up the correct 
user-space page protection. But now we're moving stuff around so we can't.

/Thomas