Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

[Xiaoyao Li]

On 6/19/2025 4:59 PM, Xiaoyao Li wrote:
> On 6/19/2025 4:13 PM, Yan Zhao wrote:
> > On Wed, May 14, 2025 at 04:41:39PM -0700, Ackerley Tng wrote:
> > > Hello,
> > >
> > > This patchset builds upon discussion at LPC 2024 and many guest_memfd
> > > upstream calls to provide 1G page support for guest_memfd by taking
> > > pages from HugeTLB.
> > >
> > > This patchset is based on Linux v6.15-rc6, and requires the mmap support
> > > for guest_memfd patchset (Thanks Fuad!) [1].
> > >
> > > For ease of testing, this series is also available, stitched together,
> > > at https://github.com/googleprodkernel/linux-cc/tree/gmem-1g-page- 
> > > support-rfc-v2
> > Just to record a found issue -- not one that must be fixed.
> >
> > In TDX, the initial memory region is added as private memory during 
> > TD's build
> > time, with its initial content copied from source pages in shared memory.
> > The copy operation requires simultaneous access to both shared source 
> > memory
> > and private target memory.
> >
> > Therefore, userspace cannot store the initial content in shared memory 
> > at the
> > mmap-ed VA of a guest_memfd that performs in-place conversion between 
> > shared and
> > private memory. This is because the guest_memfd will first unmap a PFN 
> > in shared
> > page tables and then check for any extra refcount held for the shared 
> > PFN before
> > converting it to private.
>
> I have an idea.
>
> If I understand correctly, the KVM_GMEM_CONVERT_PRIVATE of in-place 
> conversion unmap the PFN in shared page tables while keeping the content 
> of the page unchanged, right?
>
> So KVM_GMEM_CONVERT_PRIVATE can be used to initialize the private memory 
> actually for non-CoCo case actually, that userspace first mmap() it and 
> ensure it's shared and writes the initial content to it, after it 
> userspace convert it to private with KVM_GMEM_CONVERT_PRIVATE.
>
> For CoCo case, like TDX, it can hook to KVM_GMEM_CONVERT_PRIVATE if it 
> wants the private memory to be initialized with initial content, and 
> just do in-place TDH.PAGE.ADD in the hook.

And maybe a new flag for KVM_GMEM_CONVERT_PRIVATE for user space to 
explicitly request that the page range is converted to private and the 
content needs to be retained. So that TDX can identify which case needs 
to call in-place TDH.PAGE.ADD.