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Documentation/x86: Add documentation for TDX's Dynamic PAMT

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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.

Some of the docs verbiage was provided by Dave Hansen (see link). AI was
used under supervision to review the docs.

Based on a patch originally by Kiryl Shutsemau.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Link: https://lore.kernel.org/all/8e40862e-891a-4cad-8bb8-06ad25ad6061@intel.com/
Link: https://patch.msgid.link/20260904215841.303070-11-rick.p.edgecombe@intel.com
This commit is contained in:
Rick Edgecombe 2026-09-04 14:58:40 -07:00 committed by Dave Hansen
parent 1e3d791353
commit e61c837817
1 changed files with 21 additions and 0 deletions
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21
Documentation/arch/x86/tdx.rst
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@ -200,6 +200,27 @@ reflects the TCB of the currently running TDX module and therefore
changes after an update. By contrast, TEE_TCB_SVN reflects the TCB at TD
launch time and is not affected.
Dynamic PAMT
------------
The Physical Address Metadata Table (PAMT) is metadata in which the TDX
module keeps data about each physical page (think struct page). Space
for it is allocated by the VMM, consumes up to about 0.4% of system
memory and needs to be supplied to the TDX module when the TDX module is
first loaded.
Dynamic PAMT is an add-on feature that allows a VMM to dynamically
allocate the part of the PAMT which tracks 4KB pages. This reduces the
amount of memory that TDX consumes 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: TDX-Module initialized
Dynamic PAMT is enabled automatically if supported.
TDX Interaction to Other Kernel Components
------------------------------------------