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

[Dave Hansen <dave.hansen@intel.com>]

This whole thing looks like a fascinating collection of hacks. :)

ttm is taking a stack-alllocated "VMA" and handing it to vmf_insert_*()
which obviously are expecting "real" VMAs that are linked into the mm.
It's extracting some pgprot_t information from the real VMA, making a
psuedo-temporary VMA, then passing the temporary one back into the
insertion functions:

> static vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
> {
...
>         struct vm_area_struct cvma;
...
>                 if (vma->vm_flags & VM_MIXEDMAP)
>                         ret = vmf_insert_mixed(&cvma, address,
>                                         __pfn_to_pfn_t(pfn, PFN_DEV));
>                 else
>                         ret = vmf_insert_pfn(&cvma, address, pfn);

I can totally see why this needs new exports.  But, man, it doesn't seem
like something we want to keep *feeding*.

The real problem here is that the encryption bits from the device VMA's
"true" vma->vm_page_prot don't match the ones that actually get
inserted, probably because the device ptes need the encryption bits
cleared but the system memory PTEs need them set *and* they're mixed
under one VMA.

The thing we need to stop is having mixed encryption rules under one VMA.