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

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 31, 2024, Alex Bennée wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > 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 :-/).
> 
> I've applied this series to the latest 6.9.x while attempting to
> diagnose some of the virtio-gpu problems it may or may not address.
> However launching KVM guests keeps triggering a bunch of BUGs that
> eventually leave a hung guest:

Can you give v12 (which is comically large) a spin?  I still need to do more
testing, but if it too is buggy, I definitely want to know sooner than later.

Thanks!

https://lore.kernel.org/all/20240726235234.228822-1-seanjc@google.com