Re: [PATCH v9 04/11] x86/virt/tdx: Allocate refcounts for Dynamic PAMT memory

[Vishal Annapurve]

On Wed, Aug 5, 2026 at 7:09 PM Rick Edgecombe
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> The PAMT memory holds metadata for all possible TDX protected memory. Each
> physical address range is covered by PAMT entries at three levels (1GB,
> 2MB, 4KB). With Dynamic PAMT, the 4KB level of PAMT is allocated on
> demand. The kernel supplies the TDX module with page pairs to store the
> 4KB level entries, which cover 2MB of host physical memory. The kernel must
> provide this page pair before using pages from the range for TDX. If this
> is not done, SEAMCALLs that give the pages to be protected by the TDX
> module will fail.
>
> Allocate reference counters for every 2MB range to track TDX memory usage.
> This can be used to handle concurrent get/put callers, in order to
> accurately determine when the dynamic 4KB level of Dynamic PAMT needs to
> be allocated and when it can be freed.
>
> This allocation will currently consume 2MB for every 1TB of address
> space from 0 to max_pfn. The allocation size will depend on how the RAM is
> physically laid out. In a worst case scenario where the entire 52 bit
> address space is covered this would be 8GB. Then the Dynamic PAMT refcount
> allocations could hypothetically cause the savings from Dynamic PAMT to go
> negative on exotic platforms with sparse, small amounts of memory.
>
> Future changes could reduce this refcount overhead to be only allocating
> refcounts for physical ranges that contain memory that TDX can use.
> However, this is left for future work.
>
> AI was used under supervision to collect/apply feedback, review code and
> workshop logs.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Tested-by: Hongyu Ning <hongyu.ning@xxxxxxxxxxxxxxx>
> Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Reviewed-by: Tony Lindgren <tony.lindgren@xxxxxxxxxxxxxxx>
> Acked-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>

Reviewed-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>