Re: [RFC PATCH v2 02/23] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_page_demote()

[Yan Zhao]

On Fri, Nov 14, 2025 at 05:14:03PM +0800, Binbin Wu wrote:
> 
> 
> On 11/12/2025 4:06 PM, Yan Zhao wrote:
> > On Tue, Nov 11, 2025 at 05:15:22PM +0800, Huang, Kai wrote:
> > > On Mon, 2025-09-01 at 17:08 +0800, Yan Zhao wrote:
> > > > > > Do not handle TDX_INTERRUPTED_RESTARTABLE because SEAMCALL
> > > > > > TDH_MEM_PAGE_DEMOTE does not check interrupts (including NMIs) for basic
> > > > > > TDX (with or without Dynamic PAMT).
> > > > > The cover letter mentions that there is a new TDX module in planning, which
> > > > > disables the interrupt checking. I guess TDX module would need to have a
> > > > > interface to report the change, KVM then decides to enable huge page support or
> > > > > not for TDs?
> > > > Yes. But I guess detecting TDX module version or if it supports certain feature
> > > > is a generic problem. e.g., certain versions of TDX module have bugs in
> > > > zero-step mitigation and may block vCPU entering.
> > > > 
> > > > So, maybe it deserves a separate series?
> > > Looking at the spec (TDX module ABI spec 348551-007US), is it enumerated via
> > > TDX_FEATURES0.ENHANCED_DEMOTE_INTERRUPTIBILITY?
> > Yes. I checked the unreleased TDX module code that enumerates this bit (starting
> > from version TDX_1.5.28.00.972). TDH.MEM.PAGE.DEMOTE will not return
> > TDX_INTERRUPTED_RESTARTABLE for L1 VMs.
> 
> According to the content pasted by Kai below, it just says there will be no
> TDX_INTERRUPTED_RESTARTABLE for TDH.MEM.PAGE.DEMOTE if no L2 VMs.
> 
> KVM doesn't support TD partition yet, just for clarification,  what if the
> demotion is for L1 VM, but there are L2 VMs configured?
Right. The description pasted by Kai is more accurate:
"There will be no TDX_INTERRUPTED_RESTARTABLE for TDH.MEM.PAGE.DEMOTE if no L2
 VMs".

>From the code, DEMOTE may return TDX_INTERRUPTED_RESTARTABLE if
tdcs_ptr->management_fields.num_l2_vms is non-zero.

Thanks for flagging this.

> > >    5.4.25.3.9.
> > > 
> > >    Interruptibility
> > > 
> > >    If the TD is not partitioned (i.e., it has been configured with no L2
> > >    VMs), and the TDX Module enumerates
> > >    TDX_FEATURES0.ENHANCED_DEMOTE_INTERRUPTIBILITY as 1, TDH.MEM.PAGE.DEMOTE
> > >    is not interruptible.
> > > 
> > > So if the decision is to not use 2M page when TDH_MEM_PAGE_DEMOTE can return
> > > TDX_INTERRUPTED_RESTARTABLE, maybe we can just check this enumeration in
> > > fault handler and always make mapping level as 4K?
> > Thanks for this info! I think this is a very good idea and the right direction.
> > If no objection, I'll update the code in this way.
> > 
> > 
> > 
>