Re: [PATCH v7 05/11] x86/virt/tdx: Handle multiple callers in tdx_pamt_get/put()

On 7/18/26 04:44, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <[email protected]>
> 
> 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 <[email protected]>
> Co-developed-by: Rick Edgecombe <[email protected]>
> Signed-off-by: Rick Edgecombe <[email protected]>
> Reviewed-by: Binbin Wu <[email protected]>
> Reviewed-by: Chao Gao <[email protected]>
> Reviewed-by: Yan Zhao <[email protected]>
> Reviewed-by: Tony Lindgren <[email protected]>


Reviewed-by: Nikolay Borisov <[email protected]>