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

[Vishal Annapurve <vannapurve@google.com>]

On Mon, Jun 30, 2025 at 10:26 PM Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> On Mon, Jun 30, 2025 at 07:14:07AM -0700, Vishal Annapurve wrote:
> > On Sun, Jun 29, 2025 at 8:17 PM Yan Zhao <yan.y.zhao@intel.com> wrote:
> > >
> > > On Sun, Jun 29, 2025 at 11:28:22AM -0700, Vishal Annapurve wrote:
> > > > On Thu, Jun 19, 2025 at 1:59 AM Xiaoyao Li <xiaoyao.li@intel.com> 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?
> > > >
> > > > That's correct.
> > > >
> > > > >
> > > > > 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.
> > > >
> > > > I think you mean pKVM by non-coco VMs that care about private memory.
> > > > Yes, initial memory regions can start as shared which userspace can
> > > > populate and then convert the ranges to 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.
> > > >
> > > > I think this scheme will be cleaner:
> > > > 1) Userspace marks the guest_memfd ranges corresponding to initial
> > > > payload as shared.
> > > > 2) Userspace mmaps and populates the ranges.
> > > > 3) Userspace converts those guest_memfd ranges to private.
> > > > 4) For both SNP and TDX, userspace continues to invoke corresponding
> > > > initial payload preparation operations via existing KVM ioctls e.g.
> > > > KVM_SEV_SNP_LAUNCH_UPDATE/KVM_TDX_INIT_MEM_REGION.
> > > >    - SNP/TDX KVM logic fetches the right pfns for the target gfns
> > > > using the normal paths supported by KVM and passes those pfns directly
> > > > to the right trusted module to initialize the "encrypted" memory
> > > > contents.
> > > >        - Avoiding any GUP or memcpy from source addresses.
> > > One caveat:
> > >
> > > when TDX populates the mirror root, kvm_gmem_get_pfn() is invoked.
> > > Then kvm_gmem_prepare_folio() is further invoked to zero the folio.
> >
> > Given that confidential VMs have their own way of initializing private
> > memory, I think zeroing makes sense for only shared memory ranges.
> > i.e. something like below:
> > 1) Don't zero at allocation time.
> > 2) If faulting in a shared page and its not uptodate, then zero the
> > page and set the page as uptodate.
> > 3) Clear uptodate flag on private to shared conversion.
> > 4) For faults on private ranges, don't zero the memory.
> >
> > There might be some other considerations here e.g. pKVM needs
> > non-destructive conversion operation, which might need a way to enable
> > zeroing at allocation time only.
> >
> > On a TDX specific note, IIUC, KVM TDX logic doesn't need to clear
> > pages on future platforms [1].
> Yes, TDX does not need to clear pages on private page allocation.
> But current kvm_gmem_prepare_folio() clears private pages in the common path
> for both TDX and SEV-SNP.
>
> I just wanted to point out that it's a kind of obstacle that need to be removed
> to implement the proposed approach.
>

Proposed approach will work with 4K pages without any additional
changes. For huge pages it's easy to prototype this approach by just
disabling zeroing logic in guest mem on faulting and instead always
doing zeroing on allocation.

I would be curious to understand if we need zeroing on conversion for
Confidential VMs. If not, then the simple rule of zeroing on
allocation only will work for all usecases.