Re: [PATCH v11 0/8] KVM: allow mapping non-refcounted pages

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 29, 2024, David Stevens wrote:
> From: David Stevens <stevensd@chromium.org>
> 
> This patch series adds support for mapping VM_IO and VM_PFNMAP memory
> that is backed by struct pages that aren't currently being refcounted
> (e.g. tail pages of non-compound higher order allocations) into the
> guest.
> 
> Our use case is virtio-gpu blob resources [1], which directly map host
> graphics buffers into the guest as "vram" for the virtio-gpu device.
> This feature currently does not work on systems using the amdgpu driver,
> as that driver allocates non-compound higher order pages via
> ttm_pool_alloc_page().
> 
> First, this series replaces the gfn_to_pfn_memslot() API with a more
> extensible kvm_follow_pfn() API. The updated API rearranges
> gfn_to_pfn_memslot()'s args into a struct and where possible packs the
> bool arguments into a FOLL_ flags argument. The refactoring changes do
> not change any behavior.
> 
> From there, this series extends the kvm_follow_pfn() API so that
> non-refconuted pages can be safely handled. This invloves adding an
> input parameter to indicate whether the caller can safely use
> non-refcounted pfns and an output parameter to tell the caller whether
> or not the returned page is refcounted. This change includes a breaking
> change, by disallowing non-refcounted pfn mappings by default, as such
> mappings are unsafe. To allow such systems to continue to function, an
> opt-in module parameter is added to allow the unsafe behavior.
> 
> This series only adds support for non-refcounted pages to x86. Other
> MMUs can likely be updated without too much difficulty, but it is not
> needed at this point. Updating other parts of KVM (e.g. pfncache) is not
> straightforward [2].

FYI, on the off chance that someone else is eyeballing this, I am working on
revamping this series.  It's still a ways out, but I'm optimistic that we'll be
able to address the concerns raised by Christoph and Christian, and maybe even
get KVM out of the weeds straightaway (PPC looks thorny :-/).