linux-kernel - Re: [PATCH v4 16/16] Documentation/x86: Add documentation for TDX's Dynamic PAMT

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Message-ID: <96775d7e-e0d9-404e-bdb0-8c95db7f09cf@linux.intel.com>
Date: Wed, 26 Nov 2025 18:33:24 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: bp@...en8.de, chao.gao@...el.com, dave.hansen@...el.com,
 isaku.yamahata@...el.com, kai.huang@...el.com, kas@...nel.org,
 kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, mingo@...hat.com, pbonzini@...hat.com,
 seanjc@...gle.com, tglx@...utronix.de, vannapurve@...gle.com,
 x86@...nel.org, yan.y.zhao@...el.com, xiaoyao.li@...el.com,
 binbin.wu@...el.com
Subject: Re: [PATCH v4 16/16] Documentation/x86: Add documentation for TDX's
 Dynamic PAMT



On 11/21/2025 8:51 AM, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>
> Expand TDX documentation to include information on the Dynamic PAMT
> feature.
>
> The new section explains PAMT support in the TDX module and how Dynamic
> PAMT affects the kernel memory use.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> [Add feedback, update log]
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
> v3:
>   - Trim down docs to be about things that user cares about, instead
>     of development history and other details like this.
> ---
>   Documentation/arch/x86/tdx.rst | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
>
> diff --git a/Documentation/arch/x86/tdx.rst b/Documentation/arch/x86/tdx.rst
> index 61670e7df2f7..8d45d31fee29 100644
> --- a/Documentation/arch/x86/tdx.rst
> +++ b/Documentation/arch/x86/tdx.rst
> @@ -99,6 +99,27 @@ initialize::
>   
>     [..] virt/tdx: module initialization failed ...
>   
> +Dynamic PAMT
> +------------
> +
> +PAMT is memory that the TDX module needs to keep data about each page
> +(think like struct page). It needs to handed to the TDX module for its
                                         ^
                                         be
> +exclusive use. For normal PAMT, this is installed when the TDX module
> +is first loaded and comes to about 0.4% of system memory.
> +
> +Dynamic PAMT is a TDX feature that allows VMM to allocate part of the
> +PAMT as needed (the parts for tracking 4KB size pages). The other page
> +sizes (1GB and 2MB) are still allocated statically at the time of
> +TDX module initialization. This reduces the amount of memory that TDX
> +uses while TDs are not in use.
> +
> +When Dynamic PAMT is in use, dmesg shows it like:
> +  [..] virt/tdx: Enable Dynamic PAMT
> +  [..] virt/tdx: 10092 KB allocated for PAMT
> +  [..] virt/tdx: module initialized
> +
> +Dynamic PAMT is enabled automatically if supported.
> +
>   TDX Interaction to Other Kernel Components
>   ------------------------------------------
>