[PATCH v5 4/4] KVM: mmu: remove over-aggressive warnings

[David Stevens]

On Thu, Jan 6, 2022 at 4:19 AM Sean Christopherson <seanjc at google.com> wrote:
>
> On Wed, Jan 05, 2022, Sean Christopherson wrote:
> > Ah, I got royally confused by ensure_pfn_ref()'s comment
> >
> >   * Certain IO or PFNMAP mappings can be backed with valid
> >   * struct pages, but be allocated without refcounting e.g.,
> >   * tail pages of non-compound higher order allocations, which
> >   * would then underflow the refcount when the caller does the
> >   * required put_page. Don't allow those pages here.
> >                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > that doesn't apply here because kvm_faultin_pfn() uses the low level
> > __gfn_to_pfn_page_memslot().
>
> On fifth thought, I think this is wrong and doomed to fail.  By mapping these pages
> into the guest, KVM is effectively saying it supports these pages.  But if the guest
> uses the corresponding gfns for an action that requires KVM to access the page,
> e.g. via kvm_vcpu_map(), ensure_pfn_ref() will reject the access and all sorts of
> bad things will happen to the guest.
>
> So, why not fully reject these types of pages?  If someone is relying on KVM to
> support these types of pages, then we'll fail fast and get a bug report letting us
> know we need to properly support these types of pages.  And if not, then we reduce
> KVM's complexity and I get to keep my precious WARN :-)

Our current use case here is virtio-gpu blob resources [1]. Blob
resources are useful because they avoid a guest shadow buffer and the
associated memcpys, and as I understand it they are also required for
virtualized vulkan.

One type of blob resources requires mapping dma-bufs allocated by the
host directly into the guest. This works on Intel platforms and the
ARM platforms I've tested. However, the amdgpu driver sometimes
allocates higher order, non-compound pages via ttm_pool_alloc_page.
These are the type of pages which KVM is currently rejecting. Is this
something that KVM can support?

+olv, who has done some of the blob resource work.

[1] https://patchwork.kernel.org/project/dri-devel/cover/20200814024000.2485-1-gurchetansingh@chromium.org/

-David