Re: [PATCH v6 04/11] x86/virt/tdx: Allocate ref counts for Dynamic PAMT memory

[Binbin Wu]

On 5/26/2026 10:35 AM, Rick Edgecombe 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 range of PAMT is allocated on
> demand. The kernel supplies the TDX module with page pairs to store the
> 4KB 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 2 MB for every 1 TB 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 DPAMT refcount
> allocations could hypothetically cause the savings from Dynamic PAMT to go
> negative on exotic platforms with sparse, small amounts of memory.

Nit:
Do you think it make sense to print the memory size allocated for
DPAMT refcounts in the kernel log?

> 
> 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.
> 
> Assisted-by: Sashiko:claude-opus-4-6 GitHub Copilot:claude-opus-4-6 Sashiko:claude-opus-4-6
> 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>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>