Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

[Kiryl Shutsemau]

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