linux-kernel - Re: [PATCH v3 22/24] x86/tdx: Add/Remove DPAMT pages for guest private memory to demote

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Message-ID: <aW4RefKjby5Z8GWJ@yzhao56-desk.sh.intel.com>
Date: Mon, 19 Jan 2026 19:11:53 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v3 22/24] x86/tdx: Add/Remove DPAMT pages for guest
 private memory to demote

On Mon, Jan 19, 2026 at 06:52:46PM +0800, Huang, Kai wrote:
> On Tue, 2026-01-06 at 18:24 +0800, Yan Zhao wrote:
> >  u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *new_sept_page,
> > +			struct tdx_prealloc *prealloc,
> >  			u64 *ext_err1, u64 *ext_err2)
> >  {
> > -	struct tdx_module_args args = {
> > -		.rcx = gpa | level,
> > -		.rdx = tdx_tdr_pa(td),
> > -		.r8 = page_to_phys(new_sept_page),
> > +	bool dpamt = tdx_supports_dynamic_pamt(&tdx_sysinfo) && level == TDX_PS_2M;
> 
> The spec of TDH.MEM.PAGE.DEMOTE says:
> 
>   If the TDX Module is configured to use Dynamic PAMT and the large page
>   level is 1 (2MB), R12 contains the host physical address of a new 
>   PAMT page (HKID bits must be 0).
> 
> It says "... is configured to use Dynamic PAMT ...", but not ".. Dynamic
> PAMT is supported ..".
Good catch.

> tdx_supports_dynamic_pamt() only reports whether the module "supports"
> DPAMT.  Although in the DPAMT series the kernel always enables DPAMT when
> it is supported, I think it's better to have a comment point out this fact
> so we don't need to go to that series to figure out.
Will add the comment. Thanks!

> > +	u64 guest_memory_pamt_page[MAX_TDX_ARG_SIZE(r12)];
> > +	struct tdx_module_array_args args = {
> > +		.args.rcx = gpa | level,
> > +		.args.rdx = tdx_tdr_pa(td),
> > +		.args.r8 = page_to_phys(new_sept_page),
> >  	};
> >  	u64 ret;
> >  
> >  	if (!tdx_supports_demote_nointerrupt(&tdx_sysinfo))
> >  		return TDX_SW_ERROR;
> >  
> > +	if (dpamt) {
> > +		u64 *args_array = dpamt_args_array_ptr_r12(&args);
> > +
> > +		if (alloc_pamt_array(guest_memory_pamt_page, prealloc))
> > +			return TDX_SW_ERROR;
> > +
> > +		/*
> > +		 * Copy PAMT page PAs of the guest memory into the struct per the
> > +		 * TDX ABI
> > +		 */
> > +		memcpy(args_array, guest_memory_pamt_page,
> > +		       tdx_dpamt_entry_pages() * sizeof(*args_array));
> > +	}