Re: [PATCH v9 05/11] x86/virt/tdx: Handle multiple callers in tdx_pamt_get/put()

[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>
>
> tdx_pamt_get()/tdx_pamt_put() unconditionally add or remove Dynamic PAMT
> backing for the 2MB region covering the passed page. However, multiple
> callers can add or remove 4KB pages that fall within the same 2MB region
> and in that scenario only a single PAMT entry is required.
>
> Make the helpers handle only adding/removing Dynamic PAMT backing when
> required, by refcounting each 2MB range. Gate the actual Dynamic PAMT add
> and remove on refcount transitions (0->1 and 1->0). Serialize the refcount
> check and SEAMCALL with a global spinlock so the read-decide-act sequence
> is atomic. This also avoids TDX module BUSY errors, as the Dynamic PAMT add
> and remove SEAMCALLs take internal TDX module locks for the 2MB ranges of
> the specified PFN and the PAMT page pair PFNs. So simultaneous attempts on
> the same 2MB ranges of the PFNs would otherwise encounter an error, which
> would not be handleable in the put case.
>
> The lock is global and heavyweight. Use simple conditional logic to keep
> correctness obvious. This will be optimized in a later change.
>
> The pamt_refcounts[] are atomic_t's. They do not strictly need to be
> because all access is protected by pamt_lock. The overhead of an atomic_t
> in this situation is minuscule compared to the global lock. Leave the
> atomic_t in place to enable future optimization with minimal churn.
>
> AI was used under supervision to collect/apply feedback, split patches,
> 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>
> Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> Acked-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>

Reviewed-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>