On Tue, Aug 12, 2025 at 09:15:16AM -0700, Sean Christopherson wrote: > On Tue, Aug 12, 2025, Rick P Edgecombe wrote: > > On Tue, 2025-08-12 at 09:04 +0100, kas@xxxxxxxxxx wrote: > > > > > E.g. for things like TDCS pages and to some extent non-leaf S-EPT > > > > > pages, on-demand PAMT management seems reasonable. But for PAMTs that > > > > > are used to track guest-assigned memory, which is the vaaast majority > > > > > of PAMT memory, why not hook guest_memfd? > > > > > > > > This seems fine for 4K page backing. But when TDX VMs have huge page > > > > backing, the vast majority of private memory memory wouldn't need PAMT > > > > allocation for 4K granularity. > > > > > > > > IIUC guest_memfd allocation happening at 2M granularity doesn't > > > > necessarily translate to 2M mapping in guest EPT entries. If the DPAMT > > > > support is to be properly utilized for huge page backings, there is a > > > > value in not attaching PAMT allocation with guest_memfd allocation. > > I don't disagree, but the host needs to plan for the worst, especially since the > guest can effectively dictate the max page size of S-EPT mappings. AFAIK, there > are no plans to support memory overcommit for TDX guests, so unless a deployment > wants to roll the dice and hope TDX guests will use hugepages for N% of their > memory, the host will want to reserve 0.4% of guest memory for PAMTs to ensure > it doesn't unintentionally DoS the guest with an OOM condition. > > Ditto for any use case that wants to support dirty logging (ugh), because dirty > logging will require demoting all of guest memory to 4KiB mappings. > > > > Right. > > > > > > It also requires special handling in many places in core-mm. Like, what > > > happens if THP in guest memfd got split. Who would allocate PAMT for it? > > guest_memfd? I don't see why core-mm would need to get involved. And I definitely > don't see how handling page splits in guest_memfd would be more complicated than > handling them in KVM's MMU. > > > > Migration will be more complicated too (when we get there). > > Which type of migration? Live migration or page migration? Page migration. But I think after some reading, it can be manageable. -- Kiryl Shutsemau / Kirill A. Shutemov
