On Thu, Jun 19, 2025 at 05:18:44PM +0800, Xiaoyao Li wrote: > 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? However, whenever there's a GUP in TDX to get the source page, there will be an extra page refcount. > > 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. The conversion request here will be declined therefore. > > 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. >
