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

[Edgecombe, Rick P]

On Thu, 2026-07-02 at 15:20 +0800, Binbin Wu wrote:
> 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?

I think, no? Because it should normally be "small" on the scale of the system.
We have to draw the line somewhere.

> 
> > 
> > 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>

Thanks, and for the others.